mm, slab: simplify returning slab in __refill_objects_node()

mm, slab: simplify returning slab in __refill_objects_node()

When we return slabs to the partial list because we didn't fully refill
from them, we observe the min_partial limit when the returned slab is
empty, and discard it when over the limit. But it's unlikely for the
limit to be reached while we were refilling, and the worst outcome is to
have temporarily more free slabs on the list than necessary. So just
drop that code and simplify the function.

Link: https://patch.msgid.link/20260522-b4-refill-optimistic-return-v3-2-2ba78ec1c6ed@kernel.org
Reviewed-by: Hao Li <hao.li@linux.dev>
Tested-by: Hao Li <hao.li@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>