nfs: print fileid in lookup tracepoints

With this we can see the dentry -> inode linkage that's being
revalidated. A fileid of 0 means "negative dentry".

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>