tuntap: orphan frags before trying to set tx timestamp

sock_tx_timestamp() will clear all zerocopy flags of skb which may lead the
frags never to be orphaned. This will break guest to guest traffic when zerocopy
is enabled. Fix this by orphaning the frags before trying to set tx time stamp.

The issue were introduced by commit eda29772
(tun: Support software transmit time stamping).

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>