packet: fix panic in __packet_set_timestamp on tpacket_v3 in tx mode

When TX timestamping is in use with TPACKET_V3's TX ring, then we'll
hit the BUG() in __packet_set_timestamp() when ring buffer slot is
returned to user space via tpacket_destruct_skb(). This is due to v3
being assumed as unreachable here, but since 7f953ab2 ("af_packet:
TX_RING support for TPACKET_V3") it's not anymore. Fix it by filling
the timestamp back into the ring slot.

Fixes: 7f953ab2 ("af_packet: TX_RING support for TPACKET_V3")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>