linuxkpi: Fix __sg_alloc_table_from_pages loop

Commit 3e0856b63f 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.

Reviewed by:	bz, hselasky
Differential Revision:	https://reviews.freebsd.org/D39628
Fixes:	3e0856b63f ("linuxkpi: Fix `sg_alloc_table_from_pages()` to have the same API as Linux")

sys/compat/linuxkpi/common/include/linux/scatterlist.h

@@ -383,8 +383,6 @@ __sg_alloc_table_from_pages(struct sg_table *sgt,
 		unsigned long seg_size;
 		unsigned int j;
 
-		s = sg_next(s);
-
 		len = 0;
 		for (j = cur + 1; j < count; ++j) {
 			len += PAGE_SIZE;
@@ -398,6 +396,8 @@ __sg_alloc_table_from_pages(struct sg_table *sgt,
 		size -= seg_size;
 		off = 0;
 		cur = j;
+
+		s = sg_next(s);
 	}
 	KASSERT(s != NULL, ("s is NULL after loop in __sg_alloc_table_from_pages()"));