slab: Fix missing DEBUG_SLAB last user

Even with SLAB_RED_ZONE and SLAB_STORE_USER enabled, kernel would NOT store
redzone and last user data around allocated memory space if "arch cache line >
sizeof(unsigned long long)". As a result, last user information is unexpectedly
MISSED while dumping slab corruption log.

This fix makes sure that redzone and last user tags get stored unless the
required alignment breaks redzone's.

Signed-off-by: Shiyong Li <shi-yong.li@motorola.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>