udf: Limit sparing table size

Although UDF standard allows it, we don't support sparing table larger
than a single block. Check it during mount so that we don't try to
access memory beyond end of buffer.

Reported-by:  <syzbot+9991561e714f597095da@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>