SUNRPC: Refactor rpc_xdr_buf_init()

Clean up: there is some XDR initialization logic that is common
to the forward channel and backchannel. Move it to an XDR header
so it can be shared.

rpc_rqst::rq_buffer points to a buffer containing big-endian data.
Update its annotation as part of the clean up.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/net/sunrpc/backchannel_rqst.c b/net/sunrpc/backchannel_rqst.c
index 229956b..ac701c2 100644
--- a/net/sunrpc/backchannel_rqst.c
+++ b/net/sunrpc/backchannel_rqst.c
@@ -76,13 +76,7 @@
 	page = alloc_page(gfp_flags);
 	if (page == NULL)
 		return -ENOMEM;
-	buf->head[0].iov_base = page_address(page);
-	buf->head[0].iov_len = PAGE_SIZE;
-	buf->tail[0].iov_base = NULL;
-	buf->tail[0].iov_len = 0;
-	buf->page_len = 0;
-	buf->len = 0;
-	buf->buflen = PAGE_SIZE;
+	xdr_buf_init(buf, page_address(page), PAGE_SIZE);
 	return 0;
 }