Add barriers to transmitter locking functions.
Add barriers to transmitter locking functions.

We hit a bug that could only be explained by a barriering (fence or
code re-ordering) issue so I've added KeMemoryBarrier() calls to
handle both cases.

Signed-off-by: Paul Durrant <dba6cd059c0054f8a3c62ba902cd9d7238853ee4@citrix.com>

xenserver/win-xenvif,xenserver/win-xenvif,xenserver/win-xenvif,xenserver/win-xenvif