net: check first segment length on SW VLAN insertion

net: check first segment length on SW VLAN insertion

SW VLAN insertion relies on Ethernet addresses location in contiguous
memory (do not split across mbuf segments). There is no any formal
requirements on data location and mbuf structure which guarantee it.
So, check it explicitly to avoid corrupted packets if the condition
is violated. Typically software VLAN insertion is done on Tx prepare
stage and application will get indication that the packet is invalid
and cannot be transmitted.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>