ceph: fix connection fault con_work reentrancy problem

The messenger fault was clearing the BUSY bit, for reasons unclear.  This
made it possible for the con->ops->fault function to reopen the connection,
and requeue work in the workqueue--even though the current thread was
already in con_work.

This avoids a problem where the client busy loops with connection failures
on an unreachable OSD, but doesn't address the root cause of that problem.

Signed-off-by: Sage Weil <sage@newdream.net>