f2fs: fix a race condition in next_free_nid

The nm_i->fcnt checking is executed before spin_lock, so if another
thread delete the last free_nid from the list, the wrong nid may be
gotten.  So fix the race condition by moving the nm_i->fnct checking
into spin_lock.
Signed-off-by: NHuang, Ying <ying.huang@intel.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.h

@@ -115,9 +115,11 @@ static inline int next_free_nid(struct f2fs_sb_info *sbi, nid_t *nid)
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct free_nid *fnid;
 
-	if (nm_i->fcnt <= 0)
-		return -1;
 	spin_lock(&nm_i->free_nid_list_lock);
+	if (nm_i->fcnt <= 0) {
+		spin_unlock(&nm_i->free_nid_list_lock);
+		return -1;
+	}
 	fnid = list_entry(nm_i->free_nid_list.next, struct free_nid, list);
 	*nid = fnid->nid;
 	spin_unlock(&nm_i->free_nid_list_lock);