mount options: fix afs

Add a .show_options super operation to afs.

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

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>