linuxkpi: Add for_each_sgtable_{sg,page}
Needed by drm-kmod.
Reviewed by: bz
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36098
(cherry picked from commit 4370e9f1cfce6f43f170790be96b79f3da70be13)