nbd: Fix trace message for disconnect

NBD_CMD_DISC is a disconnect request, not a data discard request.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170811015749.20365-1-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
1 file changed