mm/slab.c (non-NUMA): Fix compile warning and clean up code

The non-NUMA case would do an unmatched "free_alien_cache()" on an alien
pointer that had never been allocated.

It might not matter from a code generation standpoint (since in the
non-NUMA case, the code doesn't actually _do_ anything), but it not only
results in a compiler warning, it's really really ugly too.

Fix the compiler warning by just having a matching dummy allocation.
That also avoids an unnecessary #ifdef in the code.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>