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 <paul.durrant@citrix.com>
Merge pull request #10 from pauldu/ca-120874

Add barriers to transmitter locking functions.

Approved-By: Ben Chalmers <ben.chalmers@citrix.com>