linuxkpi: Add dma_{un,}map_sgtable

Variant of dma_{un,}map_sg_attrs for struct sg_table.

Reviewed by: bz
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36106

(cherry picked from commit 9202c95f47c2c3be8972052dc31d540924db1447)