libceph: fix null dereference when unregistering linger requests

We should only clear r_osd if we are neither registered as a linger or a
regular request.  We may unregister as a linger while still registered as
a regular request (e.g., in reset_osd).  Incorrectly clearing r_osd there
leads to a null pointer dereference in __send_request.

Also simplify the parallel check in __unregister_request() where we just
removed r_osd_item and know it's empty.
Signed-off-by: Sage Weil <sage@newdream.net>

net/ceph/osd_client.c

@@ -837,8 +837,7 @@ static void __unregister_request(struct ceph_osd_client *osdc,
 			dout("moving osd to %p lru\n", req->r_osd);
 			__move_osd_to_lru(osdc, req->r_osd);
 		}
-		if (list_empty(&req->r_osd_item) &&
-		    list_empty(&req->r_linger_item))
+		if (list_empty(&req->r_linger_item))
 			req->r_osd = NULL;
 	}
 
@@ -883,7 +882,8 @@ static void __unregister_linger_request(struct ceph_osd_client *osdc,
 			dout("moving osd to %p lru\n", req->r_osd);
 			__move_osd_to_lru(osdc, req->r_osd);
 		}
-		req->r_osd = NULL;
+		if (list_empty(&req->r_osd_item))
+			req->r_osd = NULL;
 	}
 }