slab: use struct freelist_counters for local variables instead of struct slab

slab: use struct freelist_counters for local variables instead of struct slab

In several functions we declare local struct slab variables so we can
work with the freelist and counters fields (including the sub-counters
that are in the union) comfortably.

With struct freelist_counters containing the full counters definition,
we can now reduce the local variables to that type as we don't need the
other fields in struct slab.

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