f2fs: avoid kernel panic on corruption test

xfstests/generic/475 complains kernel warn/panic while testing corrupted disk.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -2349,7 +2349,6 @@ static int __f2fs_build_free_nids(struct f2fs_sb_info *sbi,
 
 			if (ret) {
 				up_read(&nm_i->nat_tree_lock);
-				f2fs_bug_on(sbi, !mount);
 				f2fs_err(sbi, "NAT is corrupt, run fsck to fix it");
 				return ret;
 			}