v4root: set the time-to-live for V4ROOT exports to the DEFAULT.

e_ttl is set to the default in init_exportent().
However V4ROOT exports never see init_exportent() as they are created
with dupexportent from a template.  So e_ttl does not get set and
export entries expire immediately.
This results in an upcall to mountd every time a V4ROOT directory
in accessed.

So set e_ttl in the template.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>