NFS: Clean up generic writeback tracepoints

Clean up the generic writeback tracepoints so they do pass the
full structures as arguments. Also ensure we report the number
of bytes actually written.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>