mac80211: Do not strip skb headroom on monitor frames

mac80211: Do not strip skb headroom on monitor frames

When a monitor interface is present together with other interfaces, a
received skb is copied and received on the monitor netdev. Before, the
copied skb was allocated with exactly the amount of space needed for
the radiotap header, resulting in an skb without any headroom at all
being received on the monitor netdev. With the introduction of eBPF
and XDP in the kernel, skbs may be processed by custom eBPF programs.
However, since the skb cannot be reallocated in the eBPF program, no
more data or headers can be pushed. The old code made sure the final
headroom was zero regardless of the value of NET_SKB_PAD, so increasing
that constant would have no effect.

Now we allocate monitor skb copies with a headroom of NET_SKB_PAD bytes
before the radiotap header. Monitor interfaces now behave in the same
way as other netdev interfaces that honor the NET_SKB_PAD constant.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Link: https://lore.kernel.org/r/20210628123713.2070753-1-johan.almbladh@anyfinetworks.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>