NFSD: Add an nfsd_cb_offload tracepoint

Record the arguments of CB_OFFLOAD callbacks so we can better
observe asynchronous copy-offload behavior. For example:

nfsd-995   [008]  7721.934222: nfsd_cb_offload:
        addr=192.168.2.51:0 client 6092a47c:35a43fc1 fh_hash=0x8739113a
        count=116528 status=0

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Olga Kornievskaia <kolga@netapp.com>
Cc: Dai Ngo <Dai.Ngo@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>