veth: Add updating of trans_start

Since commit 21a75f09

 ("bonding: Fix ARP monitor validation"),
the bonding ARP / ND link monitors depend on the trans_start time to
determine link availability.  NETIF_F_LLTX drivers must update trans_start
directly, which veth does not do.  This prevents use of the ARP or ND link
monitors with veth interfaces in a bond.

	Resolve this by having veth_xmit update the trans_start time.

Reported-by: Jonathan Toppins <jtoppins@redhat.com>
Tested-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Fixes: 21a75f09

 ("bonding: Fix ARP monitor validation")
Link: https://lore.kernel.org/netdev/b2fd4147-8f50-bebd-963a-1a3e8d1d9715@redhat.com/
Signed-off-by: David S. Miller <davem@davemloft.net>