linuxkpi: Fix __sg_alloc_table_from_pages loop

Commit 3e0856b63fe0e375a0951e05c2ef98bb2ebd9421 updated
__sg_alloc_table_from_pages to use the same API as linux, but modified
the loop condition when going over the pages in a sg list. Part of the
change included moving the sg_next call out of the for loop and into the
body, which causes an off by one error when traversing the list. Since
sg_next is called before the loop body it will skip the first element
and read one past the last element.

This caused panics when running PRIME with nvidia-drm as the off-by-one
issue causes a NULL dereference.

(cherry picked from commit 3f686532c9b4fd603d4ad2d3392232cff24188f4)

Commit 3f686532c9b4 tried to fix an issue with not properly starting
at the first page in the sg list to prevent a panic. This worked but
with the side effect of incrementing "s" during the final iteration
causing it to be NULL since the list had ended.
In cases non-DEBUG kernels this causes a panic with drm-5.15, since
"s" is NULL when we later pass it to sg_mark_end().
This change decouples the iteration sg from the return value so that
it is never incremented past the final page in the chain.

(cherry picked from commit 09b0401e91a92bcb58ea1873857b42f8211f660f)