net: tunnel: make skb_vlan_inet_prepare() return drop reasons

Make skb_vlan_inet_prepare return the skb drop reasons, which is just
what pskb_may_pull_reason() returns. Meanwhile, adjust all the call of
it.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>