f2fs: remove unused return value

Don't return any value without any usage.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.h

@@ -130,7 +130,7 @@ struct free_nid {
 	int state;		/* in use or not: NID_NEW or NID_ALLOC */
 };
 
-static inline int next_free_nid(struct f2fs_sb_info *sbi, nid_t *nid)
+static inline void next_free_nid(struct f2fs_sb_info *sbi, nid_t *nid)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct free_nid *fnid;
@@ -138,12 +138,11 @@ static inline int next_free_nid(struct f2fs_sb_info *sbi, nid_t *nid)
 	spin_lock(&nm_i->free_nid_list_lock);
 	if (nm_i->fcnt <= 0) {
 		spin_unlock(&nm_i->free_nid_list_lock);
-		return -1;
+		return;
 	}
 	fnid = list_entry(nm_i->free_nid_list.next, struct free_nid, list);
 	*nid = fnid->nid;
 	spin_unlock(&nm_i->free_nid_list_lock);
-	return 0;
 }
 
 /*