kmemcheck: make bitfield annotations truly no-ops when disabled

It turns out that even zero-sized struct members (int foo[0];) will affect
the struct layout, causing us in particular to lose 4 bytes in struct
sock.

This patch fixes the regression in CONFIG_KMEMCHECK=n case.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>