mm/slob: Remove unnecessary page_mapcount_reset() function call

After commit 401fb12c ("mm/sl*b: Differentiate struct slab fields by
sl*b implementations"), we can reorder fields of struct slab depending
on slab allocator.

For now, page_mapcount_reset() is called because page->_mapcount and
slab->units have same offset. But this is not necessary for struct slab.
Use unused field for units instead.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20211212065241.GA886691@odroid