libceph: complete lingering requests only once

An osd request marked to linger will be re-submitted in the event
a connection to the target osd gets dropped.  Currently, if there
is a callback function associated with a request it will be called
each time a request is submitted--which for lingering requests can
be more than once.

Change it so a request--including lingering ones--will get completed
(from the perspective of the user of the osd client) exactly once.

This resolves:
    http://tracker.ceph.com/issues/3967

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/osd_client.h

@@ -85,6 +85,7 @@ struct ceph_osd_request {
 	s32               r_reply_op_result[CEPH_OSD_MAX_OP];
 	int               r_got_reply;
 	int		  r_linger;
+	int		  r_completed;
 
 	struct ceph_osd_client *r_osdc;
 	struct kref       r_kref;