librbd: fix zero length request handling

Zero-length writes would hang because the completion was never
called. Reads would hit an assert about zero length in
Striper::file_to_exents().

Fix all of these cases by skipping zero-length extents. The completion
is created and finished when finish_adding_requests() is called. This
is slightly different from usual completions since it comes from the
same thread as the one scheduling the request, but zero-length aio
requests should never happen from things that might care about this,
like QEMU.

Writes and discards have had this bug since the beginning of
librbd. Reads might have avoided it until stripingv2 was added.

Fixes: #5469
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1632 from ceph/wip-5469

librbd: fix zero length request handling

Reviewed-by: Sage Weil <sage@inktank.com>