mount options: fix adfs

Add a .show_options super operation to adfs.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>