LinuxKPI: implement dma_sync_sg_for_{cpu, device}()

Implement dma_sync_sg_for_{cpu, device}() and
dma_sync_sgtable_for_device().
These functions are useful for my GSoC 2026 project, udmabuf.

Reviewed by: bz
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D57766