bpf: Clarify error expectations from bpf_clone_redirect

Commit 151e887d8f ("veth: Fixing transmit return status for dropped
packets") exposed the fact that bpf_clone_redirect is capable of
returning raw NET_XMIT_XXX return codes.

This is in the conflict with its UAPI doc which says the following:
"0 on success, or a negative error in case of failure."

Update the UAPI to reflect the fact that bpf_clone_redirect can
return positive error numbers, but don't explicitly define
their meaning.

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230911194731.286342-1-sdf@google.com

include/uapi/linux/bpf.h

@@ -1962,7 +1962,9 @@ union bpf_attr {
  * 		performed again, if the helper is used in combination with
  * 		direct packet access.
  * 	Return
- * 		0 on success, or a negative error in case of failure.
+ * 		0 on success, or a negative error in case of failure. Positive
+ * 		error indicates a potential drop or congestion in the target
+ * 		device. The particular positive error codes are not defined.
  *
  * u64 bpf_get_current_pid_tgid(void)
  * 	Description

tools/include/uapi/linux/bpf.h

@@ -1962,7 +1962,9 @@ union bpf_attr {
  * 		performed again, if the helper is used in combination with
  * 		direct packet access.
  * 	Return
- * 		0 on success, or a negative error in case of failure.
+ * 		0 on success, or a negative error in case of failure. Positive
+ * 		error indicates a potential drop or congestion in the target
+ * 		device. The particular positive error codes are not defined.
  *
  * u64 bpf_get_current_pid_tgid(void)
  * 	Description