Final fix for #7331 - Compound async SMB 2 requests don't work right.

Final fix for #7331 - Compound async SMB 2 requests don't work right.

Don't free the cancelled SMB2 req early, let the cancelation
function take care of it. Return a NT_STATUS_CANCELLED when
we find and cancel a request. Fix our SMB2 error returns to
correctly set the structuresize to 9, and add the expected
zero byte (see section 2.2.2 in the SMB2 spec.).

This causes Samba to pass the test program in this bug report
that W2K8R2 fails (heh heh :-). This is because we always cause
compound requests to get to a cancelation point before dealing
with a cancel request.

Jeremy.