mm: swap: drop last SWAP_MAP_SHMEM flag in batch in swap_entries_put_nr()

The SWAP_MAP_SHMEM indicates last map from shmem.  Therefore we can drop
SWAP_MAP_SHMEM in batch in similar way to drop last ref count in batch.

Link: https://lkml.kernel.org/r/20250325162528.68385-6-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>