libceph: unlink from o_linger_requests when clearing r_osd

Requests have to be unlinked from both osd->o_requests (normal
requests) and osd->o_linger_requests (linger requests) lists when
clearing req->r_osd.  Otherwise __unregister_linger_request() gets
confused and we trip over a !list_empty(&osd->o_linger_requests)
assert in __remove_osd().

MON=1 OSD=1:

    # cat remove-osd.sh
    #!/bin/bash
    rbd create --size 1 test
    DEV=$(rbd map test)
    ceph osd out 0
    sleep 3
    rbd map dne/dne # obtain a new osdmap as a side effect
    rbd unmap $DEV & # will block
    sleep 3
    ceph osd in 0

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>

net/ceph/osd_client.c

@@ -1395,6 +1395,7 @@ static int __map_request(struct ceph_osd_client *osdc,
 	if (req->r_osd) {
 		__cancel_request(req);
 		list_del_init(&req->r_osd_item);
+		list_del_init(&req->r_linger_osd_item);
 		req->r_osd = NULL;
 	}