cxgb4: Use struct_group() for memcpy() region

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() in struct fw_eth_tx_pkt_vm_wr around members ethmacdst,
ethmacsrc, ethtype, and vlantci, so they can be referenced together. This
will allow memcpy() and sizeof() to more easily reason about sizes,
improve readability, and avoid future warnings about writing beyond the
end of ethmacdst.

"pahole" shows no size nor member offset changes to struct
fw_eth_tx_pkt_vm_wr. "objdump -d" shows no object code changes.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>