linuxkpi: Add pagevec implementation

Needed by drm-kmod.

Reviewed by: hselasky
Obtained from: OpenBSD
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35941

(cherry picked from commit a0c171328f6689fb88e3881b8b5ee61d813b8c6d)