libceph, ceph: split ceph_show_options()

Split ceph_show_options() into two pieces and move the piece
responsible for printing client (libceph) options into net/ceph.  This
way people adding a libceph option wouldn't have to remember to update
code in fs/ceph.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>