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>