net: ethtool: Adjust exactly ETH_GSTRING_LEN-long stats to use memcpy

Many drivers populate the stats buffer using C-String based APIs (e.g.
ethtool_sprintf() and ethtool_puts()), usually when building up the
list of stats individually (i.e. with a for() loop). This, however,
requires that the source strings be populated in such a way as to have
a terminating NUL byte in the source.

Other drivers populate the stats buffer directly using one big memcpy()
of an entire array of strings. No NUL termination is needed here, as the
bytes are being directly passed through. Yet others will build up the
stats buffer individually, but also use memcpy(). This, too, does not
need NUL termination of the source strings.

However, there are cases where the strings that populate the
source stats strings are exactly ETH_GSTRING_LEN long, and GCC
15's -Wunterminated-string-initialization option complains that the
trailing NUL byte has been truncated. This situation is fine only if the
driver is using the memcpy() approach. If the C-String APIs are used,
the destination string name will have its final byte truncated by the
required trailing NUL byte applied by the C-string API.

For drivers that are already using memcpy() but have initializers that
truncate the NUL terminator, mark their source strings as __nonstring to
silence the GCC warnings.

For drivers that have initializers that truncate the NUL terminator and
are using the C-String APIs, switch to memcpy() to avoid destination
string truncation and mark their source strings as __nonstring to silence
the GCC warnings. (Also introduce ethtool_cpy() as a helper to make this
an easy replacement).

Specifically the following warnings were investigated and addressed:

../drivers/net/ethernet/chelsio/cxgb/cxgb2.c:364:9: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
364 | "TxFramesAbortedDueToXSCollisions",
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/freescale/enetc/enetc_ethtool.c:165:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
165 | { ENETC_PM_R1523X(0), "MAC rx 1523 to max-octet packets" },
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/freescale/enetc/enetc_ethtool.c:190:33: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
190 | { ENETC_PM_T1523X(0), "MAC tx 1523 to max-octet packets" },
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/google/gve/gve_ethtool.c:76:9: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
76 | "adminq_dcfg_device_resources_cnt", "adminq_set_driver_parameter_cnt",
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c:117:53: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
117 | STMMAC_STAT(ptp_rx_msg_type_pdelay_follow_up),
| ^
../drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c:46:12: note: in definition of macro 'STMMAC_STAT'
46 | { #m, sizeof_field(struct stmmac_extra_stats, m), \
| ^
../drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:328:24: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
328 | .str = "a_mac_control_frames_transmitted",
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/ethernet/mellanox/mlxsw/spectrum_ethtool.c:340:24: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (33 chars into 32 available) [-Wunterminated-string-initialization]
340 | .str = "a_pause_mac_ctrl_frames_received",
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com> # for mlxsw
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20250416010210.work.904-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>