ng_eiface: fix kernel panic due to the racecondition in ng_eiface shutdown.

PR: 244247
Reported by: Vladislav V. Prodan <admin@support.od.ua>
Reviewed by: vmaffione, lutz_donnerhacke.de
Approved by: vmaffione (mentor)
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D24557