mount options: fix spufs

Add a .show_options super operation to spufs.

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

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>