net: adjust L2 length on soft VLAN insertion

net: adjust L2 length on soft VLAN insertion

Layer 2 length must be updated after the prepend to mbuf to keep
the length right to be used by other Tx offloads.

If the packet has tunnel encapsulation, outer_l2_len should be
updated. Otherwise l2_len should be updated.

Fixes: c974021a5949 ("ether: add soft vlan encap/decap")
Cc: stable@dpdk.org
Signed-off-by: Dilshod Urazov <dilshod.urazov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>