LinuxKPI: Convert lkpi-shrinker lock to sx.

This fixes "might_sleep() with the following non-sleepable locks held:
exclusive sleep mutex lkpi-shrinker" warnings.

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D35047

(cherry picked from commit 29d5f0c148059806d8f0d9e55acdd072994fa450)