mm, slab: move slab_memcg hooks to mm/memcontrol.c

The hooks make multiple calls to functions in mm/memcontrol.c, including
to th current_obj_cgroup() marked __always_inline. It might be faster to
make a single call to the hook in mm/memcontrol.c instead. The hooks
also don't use almost anything from mm/slub.c. obj_full_size() can move
with the hooks and cache_vmstat_idx() to the internal mm/slab.h

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>