drbd: Defer new writes when detecting conflicting writes

Before submitting a new local write request, wait for any conflicting
local or remote requests to complete.

We could assume that the new request occurred first and that the
conflicting requests overwrote it (and therefore discard the new
reques), but we know for sure that the new request occurred after the
conflicting requests and so this behavior would we weird.  We would also
end up with the wrong result if the new request is not fully contained
within the conflicting requests.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>