xprtrdma: Address sparse complaint in rpcr_to_rdmar()

With "make ARCH=x86_64 allmodconfig make C=1 CF=-D__CHECK_ENDIAN__":

linux-2.6/net/sunrpc/xprtrdma/xprt_rdma.h:273:30: warning: incorrect
  type in initializer (different base types)
linux-2.6/net/sunrpc/xprtrdma/xprt_rdma.h:273:30: expected restricted
  __be32 [usertype] *buffer
linux-2.6/net/sunrpc/xprtrdma/xprt_rdma.h:273:30:    got unsigned int
  [usertype] *rq_buffer

As far as I can tell this is a false positive.

Reported-by: kbuild-all@01.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -270,9 +270,10 @@ struct rpcrdma_req {
 static inline struct rpcrdma_req *
 rpcr_to_rdmar(struct rpc_rqst *rqst)
 {
-	struct rpcrdma_regbuf *rb = container_of(rqst->rq_buffer,
-						 struct rpcrdma_regbuf,
-						 rg_base[0]);
+	void *buffer = rqst->rq_buffer;
+	struct rpcrdma_regbuf *rb;
+
+	rb = container_of(buffer, struct rpcrdma_regbuf, rg_base);
 	return rb->rg_owner;
 }