f2fs: fix a mount fail for wrong next_scan_nid

-write_checkpoint
   -do_checkpoint
      -next_free_nid    <--- something wrong with next free nid

-f2fs_fill_super
   -build_node_manager
      -build_free_nids
          -get_current_nat_page
             -__get_meta_page   <--- attempt to access beyond end of device

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>