net: create skb_gso_validate_mac_len()

net: create skb_gso_validate_mac_len()

commit 2b16f048729bf35e6c28a40cbfad07239f9dcd90 upstream.

If you take a GSO skb, and split it into packets, will the MAC
length (L2 + L3 + L4 headers + payload) of those packets be small
enough to fit within a given length?

Move skb_gso_mac_seglen() to skbuff.h with other related functions
like skb_gso_network_seglen() so we can use it, and then create
skb_gso_validate_mac_len to do the full calculation.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 4.4: There is no GSO_BY_FRAGS case to handle, so
 skb_gso_validate_mac_len() becomes a trivial comparison. Put it inline in
 <linux/skbuff.h>.]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>