xfrm: remove VLA usage in __xfrm6_sort()

In the quest to remove all stack VLA usage removed from the kernel[1],
just use XFRM_MAX_DEPTH as already done for the "class" array. In one
case, it'll do this loop up to 5, the other caller up to 6.

[1] https://lkml.org/lkml/2018/3/7/621

Co-developed-by: Andreas Christoforou <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Acked-by: Stefano Brivio <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/ipv6/xfrm6_state.c

···
 static int
 __xfrm6_sort(void **dst, void **src, int n, int (*cmp)(void *p), int maxclass)
 {
-	int i;
+	int count[XFRM_MAX_DEPTH] = { };
 	int class[XFRM_MAX_DEPTH];
-	int count[maxclass];
-
-	memset(count, 0, sizeof(count));
+	int i;
 
 	for (i = 0; i < n; i++) {
 		int c;