net: stmmac: fix resume: calculate tso last_segment

Tao Wang reports that sometimes, after resume, stmmac can watchdog:
NETDEV WATCHDOG: CPU: x: transmit queue x timed out xx ms

When this occurs, the DMA transmit descriptors contain:
eth0: 221 [0x0000000876d10dd0]: 0x73660cbe 0x8 0x42 0xb04416a0
eth0: 222 [0x0000000876d10de0]: 0x77731d40 0x8 0x16a0 0x90000000

where descriptor 221 is the TSO header and 222 is the TSO payload.
tdes3 for descriptor 221 (0xb04416a0) has both bit 29 (first
descriptor) and bit 28 (last descriptor) set, which is incorrect.
The following packet also has bit 28 set, but isn't marked as a
first descriptor, and this causes the transmit DMA to stall.

This occurs because stmmac_tso_allocator() populates the first
descriptor, but does not set .last_segment correctly. There are two
places where this matters: one is later in stmmac_tso_xmit() where
we use it to update the TSO header descriptor. The other is in the
ring/chain mode clean_desc3() which is a performance optimisation.

Rather than using tx_q->tx_skbuff_dma[].last_segment to determine
whether the first descriptor entry is the only segment, calculate the
number of descriptor entries used. If there is only one descriptor,
then the first is also the last, so mark it as such.

Further work will be necessary to either eliminate .last_segment
entirely or set it correctly. Code analysis also indicates that a
similar issue exists with .is_jumbo. These will be the subject of
a future patch.

Reported-by: Tao Wang <tao03.wang@horizon.auto>
Fixes: c2837423cb54 ("net: stmmac: Rework TX Coalesce logic")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vhq8O-00000005N5s-0Ke5@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>