net: simplify flags for tx timestamping

This patch removes the abstraction introduced by the union skb_shared_tx in
the shared skb data.

The access of the different union elements at several places led to some
confusion about accessing the shared tx_flags e.g. in skb_orphan_try().

    http://marc.info/?l=linux-netdev&m=128084897415886&w=2

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>