reiserfs: Fix memory leak of journal device string

When a filesystem is mounted with jdev mount option, we store the
journal device name in an allocated string in superblock. However we
fail to ever free that string. Fix it.

Reported-by:  <syzbot+1c6756baf4b16b94d2a6@syzkaller.appspotmail.com>
Fixes: c3aa0776

 ("reiserfs: Properly display mount options in /proc/mounts")
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>