slab: move validate_slab_ptr() from alloc_consistency_checks() to its caller

In alloc_debug_processing() we can call validate_slab_ptr() upfront and
then don't need to recheck when alloc_consistency_checks() fails for
other reasons.

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