When processing a map update consisting of multiple incrementals, we
may end up running check_linger_pool_dne() on a lingering request that
was previously added to need_resend_linger list.  If it is concluded
that the target pool doesn't exist, the request is killed off while
still on need_resend_linger list, which leads to a crash on a NULL
lreq->osd in kick_requests():

    libceph: linger_id 18446462598732840961 pool does not exist
    BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
    IP: ceph_osdc_handle_map+0x4ae/0x870
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>