linux(4): Constify vdso install/deinstall.

In order to reduce diff between arches constify vdso install/deinstall
functions like arm64.

Reviewed by: emaste
Differential revision: https://reviews.freebsd.org/D30901
MFC after: 2 weeks

(cherry picked from commit a543556c8128c52fb044250b8f52f9702c81788d)