libceph: fix msgr backoff

With commit f363e45f we replaced a bunch of hacky workqueue mutual
exclusion logic with the WQ_NON_REENTRANT flag.  One pieces of fallout is
that the exponential backoff breaks in certain cases:

 * con_work attempts to connect.
 * we get an immediate failure, and the socket state change handler queues
   immediate work.
 * con_work calls con_fault, we decide to back off, but can't queue delayed
   work.

In this case, we add a BACKOFF bit to make con_work reschedule delayed work
next time it runs (which should be immediately).

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

include/linux/ceph/messenger.h

@@ -123,6 +123,7 @@ struct ceph_msg_pos {
 #define SOCK_CLOSED	11 /* socket state changed to closed */
 #define OPENING         13 /* open connection w/ (possibly new) peer */
 #define DEAD            14 /* dead, about to kfree */
+#define BACKOFF         15
 
 /*
  * A single connection with another host.