linuxkpi: Acquire giant when adding/removing i2c adapters

We need Giant as we run in a taskqueue_thread via linux_work.
This fix detaching amdgpu and i915kms.

Reviewed by: bz, hselasky, imp
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35478