mount options: fix befs

Add a .show_options super operation to befs.

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

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Sergey S. Kostyliov <rathamahata@php4.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>