f2fs: use nm_i->next_scan_nid as default for next_free_nid

Now, if there is no free nid in nm_i->free_nid_list, 0 may be saved
into next_free_nid of checkpoint, this may cause useless scanning for
next mount.  nm_i->next_scan_nid should be a better default value than
0.

Signed-off-by: Huang, Ying <ying.huang@intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>