xprtrdma: Eliminate unnecessary lock cycle in xprt_rdma_send_request

The rpcrdma_req is not shared yet, and its associated Send hasn't
been posted, thus RMW should be safe. There's no need for the
expense of a lock cycle here.

Fixes: 0ba6f370 ("xprtrdma: Refactor rpcrdma_deferred_completion")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>