hyperv: Fix some variable name typos in send-buffer init/revoke

The changed names are union fields with the same size, so the existing code
still works. But, we now update these variables to the correct names.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>