bpf: Clarify handling of mark and tstamp by redirect_peer

When switching network namespaces with the bpf_redirect_peer helper, the
skb->mark and skb->tstamp fields are not zeroed out like they can be on
a typical netns switch. This patch clarifies that in the helper
description.

Signed-off-by: Paul Chaignon <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Link: https://patch.msgid.link/ccc86[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/bpf.h

···
  * 		the netns switch takes place from ingress to ingress without
  * 		going through the CPU's backlog queue.
  *
+ * 		*skb*\ **->mark** and *skb*\ **->tstamp** are not cleared during
+ * 		the netns switch.
+ *
  * 		The *flags* argument is reserved and must be 0. The helper is
  * 		currently only supported for tc BPF program types at the
  * 		ingress hook and for veth and netkit target device types. The

tools/include/uapi/linux/bpf.h

···
  * 		the netns switch takes place from ingress to ingress without
  * 		going through the CPU's backlog queue.
  *
+ * 		*skb*\ **->mark** and *skb*\ **->tstamp** are not cleared during
+ * 		the netns switch.
+ *
  * 		The *flags* argument is reserved and must be 0. The helper is
  * 		currently only supported for tc BPF program types at the
  * 		ingress hook and for veth and netkit target device types. The