xprtrdma: Move fr_cid to struct rpcrdma_mr

Clean up (for several purposes):

- The MR's cid is initialized sooner so that tracepoints can show
  something reasonable even if the MR is never posted.
- The MR's res.id doesn't change so the cid won't change either.
  Initializing the cid once is sufficient.
- struct rpcrdma_frwr is going away soon.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>