linuxkpi: Add dev_emerg
Needed by drm-kmod v5.8
Reviewed by: bz
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34286
(cherry picked from commit e3f1af9b33e95b549ccc7363bd963cc50cd5b9d7)