bpf: Add TCP connection BPF callbacks

Added callbacks to BPF SOCK_OPS type program before an active
connection is intialized and after a passive or active connection is
established.

The following patch demostrates how they can be used to set send and
receive buffer sizes.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -767,6 +767,17 @@ enum {
 					 * window (in packets) or -1 if default
 					 * value should be used
 					 */
+	BPF_SOCK_OPS_TCP_CONNECT_CB,	/* Calls BPF program right before an
+					 * active connection is initialized
+					 */
+	BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB,	/* Calls BPF program when an
+						 * active connection is
+						 * established
+						 */
+	BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB,	/* Calls BPF program when a
+						 * passive connection is
+						 * established
+						 */
 };
 
 #endif /* _UAPI__LINUX_BPF_H__ */