mptcp: use MPTCPOPT_HMAC_LEN macro

Use the macro MPTCPOPT_HMAC_LEN instead of a constant in struct
mptcp_options_received.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -119,7 +119,7 @@ struct mptcp_options_received {
 	u32	token;
 	u32	nonce;
 	u64	thmac;
-	u8	hmac[20];
+	u8	hmac[MPTCPOPT_HMAC_LEN];
 	u8	join_id;
 	u8	use_map:1,
 		dsn64:1,