slab: validate slab before using it in alloc_single_from_partial()

We touch slab->freelist and slab->inuse before checking the slab pointer
is actually sane. Do that validation first, which will be safer. We can
thus also remove the check from alloc_debug_processing().

This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
alloc_single_from_partial() is only called for caches with debugging
enabled so it's acceptable.

In alloc_single_from_new_slab() we just created the struct slab and call
alloc_debug_processing() to mainly set up redzones, tracking etc, while
not really expecting the consistency checks to fail. Thus don't validate
it there.

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>