bpf: Adds field bpf_sock_ops_cb_flags to tcp_sock

Adds field bpf_sock_ops_cb_flags to tcp_sock and bpf_sock_ops. Its primary
use is to determine if there should be calls to sock_ops bpf program at
various points in the TCP code. The field is initialized to zero,
disabling the calls. A sock_ops BPF program can set it, per connection and
as necessary, when the connection is established.

It also adds support for reading and writting the field within a
sock_ops BPF program. Reading is done by accessing the field directly.
However, writing is done through the helper function
bpf_sock_ops_cb_flags_set, in order to return an error if a BPF program
is trying to set a callback that is not supported in the current kernel
(i.e. running an older kernel). The helper function returns 0 if it was
able to set all of the bits set in the argument, a positive number
containing the bits that could not be set, or -EINVAL if the socket is
not a full TCP socket.

Examples of where one could call the bpf program:

1) When RTO fires
2) When a packet is retransmitted
3) When the connection terminates
4) When a packet is sent
5) When a packet is received

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/tcp.h

@@ -335,6 +335,17 @@ struct tcp_sock {
 
 	int			linger2;
 
+
+/* Sock_ops bpf program related variables */
+#ifdef CONFIG_BPF
+	u8	bpf_sock_ops_cb_flags;  /* Control calling BPF programs
+					 * values defined in uapi/linux/tcp.h
+					 */
+#define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_cb_flags & ARG)
+#else
+#define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0
+#endif
+
 /* Receiver side RTT estimation */
 	struct {
 		u32	rtt_us;