Revert "SLUB: remove useless masking of GFP_ZERO"

This reverts commit 3811dbf6

.

The masking was not at all useless, and it was sensible.  We handle
GFP_ZERO in the caller, and passing it down to any page allocator logic
is buggy and wrong.

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