mount options: fix autofs

Add a .show_options super operation to autofs.

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

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>