NFS: Be more targeted about readdirplus use when doing lookup/revalidation

There is little point in setting NFS_INO_ADVISE_RDPLUS in nfs_lookup and
nfs_lookup_revalidate() unless a process is actually doing readdir on the
parent directory.
Furthermore, there is little point in using readdirplus if we're trying
to revalidate a negative dentry.

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>