udf: Skip parent dir link count update if corrupted

[ Upstream commit c5566903 ]

If the parent directory link count is too low (likely directory inode
corruption), just skip updating its link count as if it goes to 0 too
early it can cause unexpected issues.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>