net: ip_gre: Accept IFLA_INFO_DATA-less configuration

The fix referenced below causes a crash when an ERSPAN tunnel is created
without passing IFLA_INFO_DATA. Fix by validating passed-in data in the
same way as ipgre does.

Fixes: e1f8f78f ("net: ip_gre: Separate ERSPAN newlink / changelink callbacks")
Reported-by:  <syzbot+1b4ebf4dae4e510dd219@syzkaller.appspotmail.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>