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