block backends: adjust protocol negotiation

The changes in c/s 1168:8940ccd0a425 resulted in the native not getting
selected anymore, resulting in more involved copying of individual
requests. Adjust the negotiation code so that it'll pick the native one
again if the frontend matches.

The initialization code can be considerably shrunk by using a macro to
abstract out the code common for all variants.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/blkback/xenbus.c
+++ b/drivers/xen/blkback/xenbus.c
@@ -497,10 +497,14 @@ static int connect_ring(struct backend_i
 	protocol = xenbus_read(XBT_NIL, dev->otherend, "protocol", NULL);
 	if (IS_ERR(protocol))
 		protocol = NULL;
+#ifndef CONFIG_X86_32
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_32))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
+#endif
+#ifndef CONFIG_X86_64
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_64))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
+#endif
 	else if (0 != strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE)) {
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		kfree(protocol);
--- a/drivers/xen/blktap/xenbus.c
+++ b/drivers/xen/blktap/xenbus.c
@@ -450,10 +450,14 @@ static int connect_ring(struct backend_i
 	protocol = xenbus_read(XBT_NIL, dev->otherend, "protocol", NULL);
 	if (IS_ERR(protocol))
 		protocol = NULL;
+#ifndef CONFIG_X86_32
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_32))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
+#endif
+#ifndef CONFIG_X86_64
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_64))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
+#endif
 	else if (0 != strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE)) {
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		kfree(protocol);