fsck.f2fs: refactor build_nat_area_bitmap to speed up fsck

fsck.f2fs: refactor build_nat_area_bitmap to speed up fsck

Refactor build_nat_area_bitmap, move out lookup_nat_in_journal from the
dual loops. Instead of looking up all nids in journals, we traverse nat
journals, whose entries are used to replace the NAT entries with the
corresponding nid to reduce the time build_nat_area_bitmap costs.

For "fsck -p 1", it reduces over half of the time.

		Empty		Fragmented Entirely
32G(ms):
  original	625.8		727.3
  improved	226.3		298.2
64G(ms):
  original	1159		1637.7
  improved	515.8		676.7

Signed-off-by: Junling Zheng <zhengjunling@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>