f2fs: remove unsafe bitmap checking

proc A:                      proc B:
- writeback_sb_inodes
- __writeback_single_inode
- do_writepages
- f2fs_write_node_pages
- f2fs_balance_fs_bg         - write_checkpoint
- build_free_nids            - flush_nat_entries
- __build_free_nids          - __flush_nat_entry_set
- ra_meta_pages              - get_next_nat_page
- current_nat_addr           - set_to_next_nat
[do nat_bitmap checking]     - f2fs_change_bit

For proc A, nat_bitmap and nat_bitmap_mir would be compared without lock_op and
nm_i->nat_tree_lock, while proc B is changing nat_bitmap/nat_bitmap_ver in cp.

So it is normal for nat_bitmap/nat_bitmap diffrence under such scenario.

This patch fix this by removing the monitoring point.

[Fix: 599a09b2 f2fs: check in-memory nat version bitmap]
Signed-off-by: Hou Pengyang <houpengyang@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>