net, bonding: Disallow vlan+srcmac with XDP

The new vlan+srcmac xmit policy is not implementable with XDP since
in many cases the 802.1Q payload is not present in the packet. This
can be for example due to hardware offload or in the case of veth
due to use of skbuffs internally.

This also fixes the NULL deref with the vlan+srcmac xmit policy
reported by Jonathan Toppins by additionally checking the skb
pointer.

Fixes: a815bde5 ("net, bonding: Refactor bond_xmit_hash for use with xdp_buff")
Reported-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Reviewed-by: Jonathan Toppins <jtoppins@redhat.com>
Link: https://lore.kernel.org/r/20210812145241.12449-1-joamaki@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>