udf: Verify inode link counts before performing rename

During rename, we are updating link counts of various inodes either when
rename deletes target or when moving directory across directories.
Verify involved link counts are sane so that we don't trip warnings in
VFS.

Reported-by: syzbot+3ff7365dc04a6bcafa66@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/namei.c

diff --git a/fs/udf/namei.c b/fs/udf/namei.c
index 2be775d..2cb49b6 100644
--- a/fs/udf/namei.c
+++ b/fs/udf/namei.c
@@ -791,8 +791,18 @@ static int udf_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 			retval = -ENOTEMPTY;
 			if (!empty_dir(new_inode))
 				goto out_oiter;
+			retval = -EFSCORRUPTED;
+			if (new_inode->i_nlink != 2)
+				goto out_oiter;
 		}
+		retval = -EFSCORRUPTED;
+		if (old_dir->i_nlink < 3)
+			goto out_oiter;
 		is_dir = true;
+	} else if (new_inode) {
+		retval = -EFSCORRUPTED;
+		if (new_inode->i_nlink < 1)
+			goto out_oiter;
 	}
 	if (is_dir && old_dir != new_dir) {
 		retval = udf_fiiter_find_entry(old_inode, &dotdot_name,