ceph: move mount state enum to super.h

These flags are only used in ceph filesystem in fs/ceph, so just
move it to the place it should be.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>