Implement the show_options superblock op for befs as part of a bid to get
rid of s_options and generic_show_options() to make it easier to implement
a context-based mount where the mount options can be passed individually
over a file descriptor.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Luis de Bethencourt <luisbg@osg.samsung.com>
cc: Salah Triki <salah.triki@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>