linuxkpi: Move device_create_groups_vargs to linux_compat.c

device_create_groups_vargs encodes the size of struct device. Move
definition from .h to .c to move this size into the linuxkpi module
rather than encoding it in all client driver modules.

Sponsored by: Netflix
Reviewed by: hselasky, emaste
Differential Revision: https://reviews.freebsd.org/D34768