inet: Refactor INET_ECN_decapsulate()

Drivers that support tunnel decapsulation (IPinIP or NVE) need to
configure the underlying device to conform to the behavior outlined in
RFC 6040 with respect to the ECN bits.

This behavior is implemented by INET_ECN_decapsulate() which requires an
skb to be passed where the ECN CE bit can be potentially set. Since
these drivers do not need to mark an skb, but only configure the device
to do so, factor out the business logic to __INET_ECN_decapsulate() and
potentially perform the marking in INET_ECN_decapsulate().

This allows drivers to invoke __INET_ECN_decapsulate() and configure the
device.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Suggested-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>