slab: use struct freelist_counters as parameters in relevant functions

slab: use struct freelist_counters as parameters in relevant functions

In functions such as [__]slab_update_freelist() and
__slab_update_freelist_fast/slow() we pass old and new freelist and
counters as 4 separate parameters. The underlying
__update_freelist_fast() then constructs struct freelist_counters
variables for passing the full freelist+counter combinations to cmpxchg
double.

In most cases we actually start with struct freelist_counters variables,
but then pass the individual fields, only to construct new struct
freelist_counters variables. While it's all inlined and thus should be
efficient, we can simplify this code.

Thus replace the 4 parameters for individual fields with two pointers to
struct freelist_counters wherever applicable. __update_freelist_fast()
can then pass them directly to try_cmpxchg_freelist().

The code is also more obvious as the pattern becomes unified such that
we set up "old" and "new" struct freelist_counters variables upfront as
we fully need them to be, and simply call [__]slab_update_freelist() on
them.  Previously some of the "new" values would be hidden among the
many parameters and thus make it harder to figure out what the code
does.

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