slirp: use DIV_ROUND_UP

I used the clang-tidy qemu-round check to generate the fix:
https://github.com/elmarco/clang-tools-extra

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

slirp/ip6.h

@@ -57,9 +57,9 @@ static inline bool in6_equal_mach(const struct in6_addr *a,
                                   const struct in6_addr *b,
                                   int prefix_len)
 {
-    if (memcmp(&(a->s6_addr[(prefix_len + 7) / 8]),
-               &(b->s6_addr[(prefix_len + 7) / 8]),
-               16 - (prefix_len + 7) / 8) != 0) {
+    if (memcmp(&(a->s6_addr[DIV_ROUND_UP(prefix_len, 8)]),
+               &(b->s6_addr[DIV_ROUND_UP(prefix_len, 8)]),
+               16 - DIV_ROUND_UP(prefix_len, 8)) != 0) {
         return 0;
     }