bpf: sockmap sample support for bpf_msg_cork_bytes()

Add sample application support for the bpf_msg_cork_bytes helper. This
lets the user specify how many bytes each verdict should apply to.

Similar to apply_bytes() tests these can be run as a stand-alone test
when used without other options or inline with other tests by using
the txmsg_cork option along with any of the basic tests txmsg,
txmsg_redir, txmsg_drop.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/include/uapi/linux/bpf.h

@@ -792,7 +792,8 @@ union bpf_attr {
 	FN(override_return),		\
 	FN(sock_ops_cb_flags_set),	\
 	FN(msg_redirect_map),		\
-	FN(msg_apply_bytes),
+	FN(msg_apply_bytes),		\
+	FN(msg_cork_bytes),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call