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>