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().

Reproducer, 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 bar/b # 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>
libceph: clear r_req_lru_item in __unregister_linger_request()

kick_requests() can put linger requests on the notarget list.  This
means we need to clear the much-overloaded req->r_req_lru_item in
__unregister_linger_request() as well, or we get an assertion failure
in ceph_osdc_release_request() - !list_empty(&req->r_req_lru_item).

AFAICT the assumption was that registered linger requests cannot be on
any of req->r_req_lru_item lists, but that's clearly not the case.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
libceph: change from BUG to WARN for __remove_osd() asserts

No reason to use BUG_ON for osd request list assertions.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>