slab: turn freelist_aba_t to a struct and fully define counters there

slab: turn freelist_aba_t to a struct and fully define counters there

In struct slab we currently have freelist and counters pair, where
counters itself is a union of unsigned long with a sub-struct of
several smaller fields. Then for the usage with double cmpxchg we have
freelist_aba_t that duplicates the definition of the freelist+counters
with implicitly the same layout as the full definition in struct slab.

Thanks to -fms-extension we can now move the full counters definition to
freelist_aba_t (while changing it to struct freelist_counters as a
typedef is unnecessary and discouraged) and replace the relevant part in
struct slab to an unnamed reference to it.

The immediate benefit is the removal of duplication and no longer
relying on the same layout implicitly. It also allows further cleanups
thanks to having the full definition of counters in struct
freelist_counters.

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