DTLS: remove unused cookie field

Note that this commit constifies a user callback parameter and therefore
will break compilation for applications using this callback. But unless
they are abusing write access to the buffer, the fix is trivial.

Reviewed-by: Andy Polyakov <appro@openssl.org>

ssl/packet_locl.h

@@ -62,6 +62,7 @@
 # include <string.h>
 # include <openssl/bn.h>
 # include <openssl/buffer.h>
+# include <openssl/crypto.h>
 # include "e_os.h"
 
 # ifdef __cplusplus
@@ -124,6 +125,18 @@ static inline void PACKET_null_init(PACKET *pkt)
     pkt->remaining = 0;
 }
 
+/*
+ * Returns 1 if the packet has length |num| and its contents equal the |num|
+ * bytes read from |ptr|. Returns 0 otherwise (lengths or contents not equal).
+ * If lengths are equal, performs the comparison in constant time.
+ */
+__owur static inline int PACKET_equal(const PACKET *pkt, const void *ptr,
+                                      size_t num) {
+    if (PACKET_remaining(pkt) != num)
+        return 0;
+    return CRYPTO_memcmp(pkt->curr, ptr, num) == 0;
+}
+
 /*
  * Peek ahead and initialize |subpkt| with the next |len| bytes read from |pkt|.
  * Data is not copied: the |subpkt| packet will share its underlying buffer with