mm/slub: cleanup and repurpose some stat items

A number of stat items related to cpu slabs became unused, remove them.

Two of those were ALLOC_FASTPATH and FREE_FASTPATH. But instead of
removing those, use them instead of ALLOC_PCS and FREE_PCS, since
sheaves are the new (and only) fastpaths, Remove the recently added
_PCS variants instead.

Change where FREE_SLOWPATH is counted so that it only counts freeing of
objects by slab users that (for whatever reason) do not go to a percpu
sheaf, and not all (including internal) callers of __slab_free(). Thus
flushing sheaves (counted by SHEAF_FLUSH) no longer also increments
FREE_SLOWPATH. This matches how ALLOC_SLOWPATH doesn't count sheaf
refills (counted by SHEAF_REFILL).

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>