svcrdma: Prevent a UAF in svc_rdma_send()

In some error flow cases, svc_rdma_wc_send() releases @ctxt. Copy
the sc_cid field in @ctxt to a stack variable in order to guarantee
that the value is available after the ib_post_send() call.

In case the new comment looks a little strange, this will be done
with at least one more field in a subsequent patch.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>