bpf: avoid unused variable warning in tcp_bpf_rtt()

When CONFIG_BPF is disabled, we get a warning for an unused
variable:

In file included from drivers/target/target_core_device.c:26:
include/net/tcp.h:2226:19: error: unused variable 'tp' [-Werror,-Wunused-variable]
        struct tcp_sock *tp = tcp_sk(sk);

The variable is only used in one place, so it can be
replaced with its value there to avoid the warning.

Fixes: 23729ff2 ("bpf: add BPF_CGROUP_SOCK_OPS callback that is executed on every RTT")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/net/tcp.h

@@ -2223,9 +2223,7 @@ static inline bool tcp_bpf_ca_needs_ecn(struct sock *sk)
 
 static inline void tcp_bpf_rtt(struct sock *sk)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
-
-	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTT_CB_FLAG))
+	if (BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk), BPF_SOCK_OPS_RTT_CB_FLAG))
 		tcp_call_bpf(sk, BPF_SOCK_OPS_RTT_CB, 0, NULL);
 }