octeontx2-pf: Add error handling to VLAN unoffload handling

octeontx2-pf: Add error handling to VLAN unoffload handling

otx2_sq_append_skb makes used of __vlan_hwaccel_push_inside()
to unoffload VLANs - push them from skb meta data into skb data.
However, it omitts a check for __vlan_hwaccel_push_inside()
returning NULL.

Found by inspection based on [1] and [2].
Compile tested only.

[1] Re: [PATCH net-next v1] net: stmmac: Enable TSO on VLANs
    https://lore.kernel.org/all/ZmrN2W8Fye450TKs@shell.armlinux.org.uk/
[2] Re: [PATCH net-next v2] net: stmmac: Enable TSO on VLANs
    https://lore.kernel.org/all/CANn89i+11L5=tKsa7V7Aeyxaj6nYGRwy35PAbCRYJ73G+b25sg@mail.gmail.com/

Fixes: fd9d7859db6c ("octeontx2-pf: Implement ingress/egress VLAN offload")
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>