NFS: Record task, client ID, and XID in xdr_status trace points

When triggering an nfs_xdr_status trace point, record the task ID
and XID of the failing RPC to better pinpoint the problem.

This feels like a bit of a layering violation.

Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>