mptcp: Remove set but not used variable 'can_ack'

Fixes gcc '-Wunused-but-set-variable' warning:

net/mptcp/options.c: In function 'mptcp_established_options_dss':
net/mptcp/options.c:338:7: warning:
 variable 'can_ack' set but not used [-Wunused-but-set-variable]

commit dc093db5 ("mptcp: drop unneeded checks")
leave behind this unused, remove it.
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -335,7 +335,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	struct mptcp_sock *msk;
 	unsigned int ack_size;
 	bool ret = false;
-	bool can_ack;
 	u8 tcp_fin;
 
 	if (skb) {
@@ -364,7 +363,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 	/* passive sockets msk will set the 'can_ack' after accept(), even
 	 * if the first subflow may have the already the remote key handy
 	 */
-	can_ack = true;
 	opts->ext_copy.use_ack = 0;
 	msk = mptcp_sk(subflow->conn);
 	if (!READ_ONCE(msk->can_ack)) {