f2fs: avoid infinite loop in f2fs_alloc_nid

If we have an error in f2fs_build_free_nids, we're able to fall into a loop
to find free nids.
Suggested-by: Chao Yu <chao@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -2350,8 +2350,9 @@ bool f2fs_alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid)
 	spin_unlock(&nm_i->nid_list_lock);
 
 	/* Let's scan nat pages and its caches to get free nids */
-	f2fs_build_free_nids(sbi, true, false);
-	goto retry;
+	if (!f2fs_build_free_nids(sbi, true, false))
+		goto retry;
+	return false;
 }
 
 /*