cephx: Fix incorrect use of __le16/32/64

cephx: Fix incorrect use of __le16/32/64

Use ceph_le16/32/64 instead of __le16/32/64 (which are no-op outside
of kernel code).

Note that I've also changes cephx_calc_client_server_challenge to
use ceph_le64 instead of manually byte-swapping by mswab.  (This
is a no-op, but it seems more consistent to use the ceph_le types
throughout.)

Fixes (partially): https://tracker.ceph.com/issues/41605

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>