resize.f2fs: fix wrong nat_bits migration

resize.f2fs: fix wrong nat_bits migration

When wring the last nat_bits in new checkpoint, we call write_nat_bits()
to recover the nat_bits. But, we didn't update nm_i->nat_bitmap which we can
get the up-to-date nat blocks. After sit migration, old checkpoint has no
longer valid nat blocks, resulting in wrong free nid allocation at the following
mount time.

Let's indicate new nat_blocks by adjusting nat_bitmap in nat migration.

Reported-by: Lianjun Huang <huanglianjun@vivo.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>