ceph: avoid null deref in osd request error path

If we interrupt an osd request, we call __cancel_request, but it wasn't
verifying that req->r_osd was non-NULL before dereferencing it.  This could
cause a crash if osds were flapping and we aborted a request on said osd.

Reported-by: Henry C Chang <henry_c_chang@tcloudcomputing.com>
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/osd_client.c

@@ -549,7 +549,7 @@ static void __unregister_request(struct ceph_osd_client *osdc,
  */
 static void __cancel_request(struct ceph_osd_request *req)
 {
-	if (req->r_sent) {
+	if (req->r_sent && req->r_osd) {
 		ceph_con_revoke(&req->r_osd->o_con, req->r_request);
 		req->r_sent = 0;
 	}