apparmor: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
security/apparmor/lib.c:139:23: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/lib.c

@@ -136,7 +136,7 @@ __counted char *aa_str_alloc(int size, gfp_t gfp)
 {
 	struct counted_str *str;
 
-	str = kmalloc(sizeof(struct counted_str) + size, gfp);
+	str = kmalloc(struct_size(str, name, size), gfp);
 	if (!str)
 		return NULL;