[PATCH] md: use BUILD_BUG_ON

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>