igb: Only call skb_tx_timestamp after descriptors are ready

Currently, skb_tx_timestamp() is being called before the Tx
descriptors are prepared in igb_xmit_frame_ring(), which happens
during either the igb_tso() or igb_tx_csum() calls.

Given that now the skb->tstamp might be used to carry the timestamp
for SO_TXTIME, we must only call skb_tx_timestamp() after the
information has been copied into the Tx descriptors.

Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>