ifconfig: fix gre(4) status

Set ifr->ifr_name to display gre options
for the interface.

Reviewed by: glebius, zlei
Approved by: glebius (mentor)
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D55099

(cherry picked from commit c2ba906fa63982d7aa799466d83716d39fe91d2b)