linuxkpi: Fix sg_alloc_table_from_pages() to have the same API as Linux

It now returns a struct scatterlist * pointer instead of an error
code only.

The implementation is incomplete because it doesn't use the prv
argument.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38077

(cherry picked from commit 3e0856b63fe0e375a0951e05c2ef98bb2ebd9421)