nfsd: fix encode_entryplus_baggage stack usage

We stick an extra svc_fh in nfsd3_readdirres to save the need to
kmalloc, though maybe it would be fine to kmalloc instead.

Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>