Replace svc_rdma_post_recv() with the new batch receive mechanism.
For the moment it is posting just a single Receive WR at a time,
so no change in behavior is expected.

Since svc_rdma_wc_receive() was the last call site for
svc_rdma_post_recv(), it is removed.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>