libceph: make sure need_resend targets reflect latest map

Otherwise we may miss events like PG splits, pool deletions, etc when
we get multiple incremental maps at once.  Because check_pool_dne() can
now be fed an unlinked request, finish_request() needed to be taught to
handle unlinked requests.

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