libsnapshot: snapuserd: Handle flush request

Handle flush operations by dm-snap post merge and
the ABI changes from dm-user. This
is now in sync with the latest dm-user patch (patch 25).

In case of any failures observed in daemon in the IO
path, return error code back to dm-user which
will eventually fail the IO.

Bug: 168311203
Test: vts_libsnapshot_test, cow_snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I4af63845f8c3e1c445f6c55374ea58b6f3454795