ip_gre: check packet length and mtu correctly in erspan tx

ip_gre: check packet length and mtu correctly in erspan tx

Similarly to early patch for erspan_xmit(), the ARPHDR_ETHER device
is the length of the whole ether packet.  So skb->len should subtract
the dev->hard_header_len.

Fixes: 1a66a836da63 ("gre: add collect_md mode to ERSPAN tunnel")
Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: William Tu <[email protected]>
Cc: Xin Long <[email protected]>
Cc: David Laight <[email protected]>
Reviewed-by: Xin Long <[email protected]>
Signed-off-by: David S. Miller <[email protected]>