cifs: adjust sequence number downward after signing NT_CANCEL request

commit 31efee60f489c759c341454d755a9fd13de8c03d upstream.

When a call goes out, the signing code adjusts the sequence number
upward by two to account for the request and the response. An NT_CANCEL
however doesn't get a response of its own, it just hurries the server
along to get it to respond to the original request more quickly.
Therefore, we must adjust the sequence number back down by one after
signing a NT_CANCEL request.

Reported-by: Tim Perry <tdparmor-sambabugs@yahoo.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>