[PATCH] slab: optimize constant-size kzalloc calls

As suggested by Eric Dumazet, optimize kzalloc() calls that pass a
compile-time constant size.  Please note that the patch increases kernel
text slightly (~200 bytes for defconfig on x86).

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>