s3:smbd: fix SMB2 aio cancelling

s3:smbd: fix SMB2 aio cancelling

As we currently don't attempt to cancel the internal aio request, we
must ignore the SMB2 cancel request and continue to process the SMB2
request, cf MS-SM2 3.3.5.16:

  If the target request is not successfully canceled, processing of the
  target request MUST continue and no response is sent to the cancel
  request.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13667

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>