libceph: introduce ceph_x_authorizer_cleanup()

Commit ae385eaf ("libceph: store session key in cephx authorizer")
introduced ceph_x_authorizer::session_key, but didn't update all the
exit/error paths.  Introduce ceph_x_authorizer_cleanup() to encapsulate
ceph_x_authorizer cleanup and switch to it.  This fixes ceph_x_destroy(),
which currently always leaks key and ceph_x_build_authorizer() error
paths.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>