kernel: use skb_cow_head() to deal with cloned skbs

We need to ensure there is enough headroom to push extra header,
but we also need to check if we are allowed to change headers.

skb_cow_head() is the proper helper to deal with this.

Fixes Ethernet<->WiFi bridge for Raspberry Pi and probably other devices.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>