libceph: behave in mon_fault() if cur_mon < 0

This can happen if __close_session() in ceph_monc_stop() races with
a connection reset.  We need to ignore such faults, otherwise it's
likely we would take !hunting, call __schedule_delayed() and end up
with delayed_work() executing on invalid memory, among other things.

The (two!) con->private tests are useless, as nothing ever clears
con->private.  Nuke them.

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