libceph: un-backoff on tick when we have a authenticated session

This means that if we do some backoff, then authenticate, and are
healthy for an extended period of time, a subsequent failure won't
leave us starting our hunting sequence with a large backoff.

Mirrors ceph.git commit d466bc6e66abba9b464b0b69687cf45c9dccf383.

Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>