f2fs: optimize scan_nat_page()

When nm_i->fcnt > 2 * MAX_FREE_NIDS, stop scanning other NAT entries.

Signed-off-by: Haicheng Li <haicheng.li@linux.intel.com>
[Jaegeuk Kim: fix handling the return value of add_free_nid()]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>