bnx2x: Use struct_group() for memcpy() region

bnx2x: 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 nig_stats around members egress_mac_pkt0_lo,
egress_mac_pkt0_hi, egress_mac_pkt1_lo, and egress_mac_pkt1_hi (and the
respective members in struct bnx2x_eth_stats), 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 struct bnx2x_eth_stats's rx_stat_ifhcinbadoctets_hi.

"pahole" shows no size nor member offset changes to either struct.
"objdump -d" shows no meaningful object code changes (i.e. only source
line number induced differences and optimizations).

Additionally adds BUILD_BUG_ON() to compare the separate struct group
sizes.

Reviewed-by: Prabhakar Kushwaha <pkushwaha@marvell.com>
Link: https://lore.kernel.org/lkml/DM5PR18MB2229B0413C372CC6E49D59A3B2C59@DM5PR18MB2229.namprd18.prod.outlook.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>