NFS: Don't gratuitously clear the inode cache when lookup failed

The fact that the lookup revalidation failed, does not mean that the
inode contents have changed.

Fixes: 5ceb9d7f ("NFS: Refactor nfs_lookup_revalidate()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>