libeth: xdp: Disable generic kCFI pass for libeth_xdp_tx_xmit_bulk()

When building drivers/net/ethernet/intel/idpf/xsk.c for ARCH=arm with
CONFIG_CFI=y using a version of LLVM prior to 22.0.0, there is a
BUILD_BUG_ON failure:

$ cat arch/arm/configs/repro.config
CONFIG_BPF_SYSCALL=y
CONFIG_CFI=y
CONFIG_IDPF=y
CONFIG_XDP_SOCKETS=y

$ make -skj"$(nproc)" ARCH=arm LLVM=1 clean defconfig repro.config drivers/net/ethernet/intel/idpf/xsk.o
In file included from drivers/net/ethernet/intel/idpf/xsk.c:4:
include/net/libeth/xsk.h:205:2: error: call to '__compiletime_assert_728' declared with 'error' attribute: BUILD_BUG_ON failed: !__builtin_constant_p(tmo == libeth_xsktmo)
205 | BUILD_BUG_ON(!__builtin_constant_p(tmo == libeth_xsktmo));
| ^
...

libeth_xdp_tx_xmit_bulk() indirectly calls libeth_xsk_xmit_fill_buf()
but these functions are marked as __always_inline so that the compiler
can turn these indirect calls into direct ones and see that the tmo
parameter to __libeth_xsk_xmit_fill_buf_md() is ultimately libeth_xsktmo
from idpf_xsk_xmit().

Unfortunately, the generic kCFI pass in LLVM expands the kCFI bundles
from the indirect calls in libeth_xdp_tx_xmit_bulk() in such a way that
later optimizations cannot turn these calls into direct ones, making the
BUILD_BUG_ON fail because it cannot be proved at compile time that tmo
is libeth_xsktmo.

Disable the generic kCFI pass for libeth_xdp_tx_xmit_bulk() to ensure
these indirect calls can always be turned into direct calls to avoid
this error.

Closes: https://github.com/ClangBuiltLinux/linux/issues/2124
Fixes: 9705d6552f58 ("idpf: implement Rx path for AF_XDP")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Acked-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20251025-idpf-fix-arm-kcfi-build-error-v1-3-ec57221153ae@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>