svcrdma: Optimize svc_rdma_cc_init()

The atomic_inc_return() in svc_rdma_send_cid_init() is expensive.

Some svc_rdma_chunk_ctxt's now reside in long-lived container
structures. They don't need a fresh completion ID for every I/O
operation.

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