slab: clarify and fix calculate_slab_order()

If we triggered the 'offslab_limit' test, we would return with
cachep->gfporder incremented once too many times.

This clarifies the logic somewhat, and fixes that bug.

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

diff --cc mm/slab.c
Simple merge