net/packet: Improve the comment about LL header visibility criteria

The "dev_has_header" function, recently added in
commit d549699048b4 ("net/packet: fix packet receive on L3 devices
without visible hard header"),
is more accurate as criteria for determining whether a device exposes
the LL header to upper layers, because in addition to dev->header_ops,
it also checks for dev->header_ops->create.

When transmitting an skb on a device, dev_hard_header can be called to
generate an LL header. dev_hard_header will only generate a header if
dev->header_ops->create is present.

Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20210205224124.21345-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/packet/af_packet.c

@@ -135,11 +135,11 @@ Resume
 On transmit:
 ------------
 
-dev->header_ops != NULL
+dev_has_header(dev) == true
    mac_header -> ll header
    data       -> ll header
 
-dev->header_ops == NULL (ll header is invisible to us)
+dev_has_header(dev) == false (ll header is invisible to us)
    mac_header -> data
    data       -> data