NFSD: Add nfsd_clid_confirmed tracepoint

This replaces a dprintk call site in order to get greater visibility
on when client IDs are confirmed or re-used. Simple example:

            nfsd-995   [000]   126.622975: nfsd_compound:        xid=0x3a34e2b1 opcnt=1
            nfsd-995   [000]   126.623005: nfsd_cb_args:         addr=192.168.2.51:45901 client 60958e3b:9213ef0e prog=1073741824 ident=1
            nfsd-995   [000]   126.623007: nfsd_compound_status: op=1/1 OP_SETCLIENTID status=0
            nfsd-996   [001]   126.623142: nfsd_compound:        xid=0x3b34e2b1 opcnt=1
  >>>>      nfsd-996   [001]   126.623146: nfsd_clid_confirmed:  client 60958e3b:9213ef0e
            nfsd-996   [001]   126.623148: nfsd_cb_probe:        addr=192.168.2.51:45901 client 60958e3b:9213ef0e state=UNKNOWN
            nfsd-996   [001]   126.623154: nfsd_compound_status: op=1/1 OP_SETCLIENTID_CONFIRM status=0

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>