bpf: Add BPF_FUNC_tcp_send_ack helper

Add a helper to send out a tcp-ack.  It will be used in the later
bpf_dctcp implementation that requires to send out an ack
when the CE state changed.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200109004551.3900448-1-kafai@fb.com

include/uapi/linux/bpf.h

@@ -2837,6 +2837,14 @@ union bpf_attr {
  * 	Return
  * 		On success, the strictly positive length of the string,	including
  * 		the trailing NUL character. On error, a negative value.
+ *
+ * int bpf_tcp_send_ack(void *tp, u32 rcv_nxt)
+ *	Description
+ *		Send out a tcp-ack. *tp* is the in-kernel struct tcp_sock.
+ *		*rcv_nxt* is the ack_seq to be sent out.
+ *	Return
+ *		0 on success, or a negative error in case of failure.
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -2954,7 +2962,8 @@ union bpf_attr {
 	FN(probe_read_user),		\
 	FN(probe_read_kernel),		\
 	FN(probe_read_user_str),	\
-	FN(probe_read_kernel_str),
+	FN(probe_read_kernel_str),	\
+	FN(tcp_send_ack),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call