xprtrdma: Add an rpcrdma_mr_completion_class

I found it confusing that the MR_EVENT class displays the mr.id but
the associated COMPLETION_EVENT class displays a cid (that happens
to contain the mr.id!). To make it a little easier on humans who
have to read and interpret these events, create an MR_COMPLETION
class that displays the mr.id in the same way as the MR_EVENT class.

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