LinuxKPI: Implement sequence counters and sequential locks

as a thin wrapper around native version found in sys/seqc.h.
This replaces out-of-base GPLv2-licensed code used by drm-kmod.

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D31006

(cherry picked from commit 8b33cb8303d3d7ff12e34327a872ef9fdb29eab9)