mount options: fix capifs

Add a .show_options super operation to capifs.

Use generic_show_options() and save the complete option string in
capifs_remount().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>