xprtrdma: rpcrdma_bc_receive_call() should init rq_private_buf.len

Some NFSv4.1 OPEN requests were hanging waiting for the NFS server
to finish recalling delegations. Turns out that each NFSv4.1 CB
request on RDMA gets a GARBAGE_ARGS reply from the Linux client.

Commit 756b9b37 added a line in bc_svc_process that
overwrites the incoming rq_rcv_buf's length with the value in
rq_private_buf.len. But rpcrdma_bc_receive_call() does not invoke
xprt_complete_bc_request(), thus rq_private_buf.len is not
initialized. svc_process_common() is invoked with a zero-length
RPC message, and fails.

Fixes: 756b9b37

 ('SUNRPC: Fix callback channel')
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>