linuxkpi: Mitigate a seqlock livelock

Disable preemption in seqlock write sections when using the _irqsave
variant. This ensures that a writer can't be preempted and subsequently
starved by a reader running in a callout handler on the same CPU.

This fixes occasional display hangs seen when using the i915 driver.

Tested by: emaste, wulf
Reviewed by: wulf, hselasky
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35021