libceph: fix linger request requeuing

The r_req_lru_item list node moves between several lists, and that cycle
is not directly related (and does not begin) with __register_request().
Initialize it in the request constructor, not __register_request(). This
fixes later badness (below) when OSDs restart underneath an rbd mount.

Crashes we've seen due to this include:

[  213.974288] kernel BUG at net/ceph/messenger.c:2193!

and

[  144.035274] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
[  144.035278] IP: [<ffffffffa036c053>] con_work+0x1463/0x2ce0 [libceph]
Signed-off-by: Sage Weil <sage@newdream.net>

net/ceph/osd_client.c

@@ -217,6 +217,7 @@ struct ceph_osd_request *ceph_osdc_alloc_request(struct ceph_osd_client *osdc,
 	INIT_LIST_HEAD(&req->r_unsafe_item);
 	INIT_LIST_HEAD(&req->r_linger_item);
 	INIT_LIST_HEAD(&req->r_linger_osd);
+	INIT_LIST_HEAD(&req->r_req_lru_item);
 	req->r_flags = flags;
 
 	WARN_ON((flags & (CEPH_OSD_FLAG_READ|CEPH_OSD_FLAG_WRITE)) == 0);
@@ -816,13 +817,10 @@ static void __register_request(struct ceph_osd_client *osdc,
 {
 	req->r_tid = ++osdc->last_tid;
 	req->r_request->hdr.tid = cpu_to_le64(req->r_tid);
-	INIT_LIST_HEAD(&req->r_req_lru_item);
-
 	dout("__register_request %p tid %lld\n", req, req->r_tid);
 	__insert_request(osdc, req);
 	ceph_osdc_get_request(req);
 	osdc->num_requests++;
-
 	if (osdc->num_requests == 1) {
 		dout(" first request, scheduling timeout\n");
 		__schedule_osd_timeout(osdc);