ibmvscsi: Add memory barriers for send / receive

ibmvscsi: Add memory barriers for send / receive

commit 7114aae02742d6b5c5a0d39a41deb61d415d3717 upstream.

Add a memory barrier prior to sending a new command to the VIOS
to ensure the VIOS does not receive stale data in the command buffer.
Also add a memory barrier when processing the CRQ for completed commands.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[bwh: Backported to 3.2: as the iSeries code is still present, these
 functions have different names and live in rpa_vscsi.c.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>