xprtrdma: Fix helper that drains the transport

We want to drain only the RQ first. Otherwise the transport can
deadlock on ->close if there are outstanding Send completions.

Fixes: 6d2d0ee2 ("xprtrdma: Replace rpcrdma_receive_wq ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>