vti6: remove !skb->ignore_df check from vti6_xmit()

Before the commit d6990976 ("vti6: fix PMTU caching and reporting
on xmit") '!skb->ignore_df' check was always true because the function
skb_scrub_packet() was called before it, resetting ignore_df to zero.

In the commit, skb_scrub_packet() was moved below, and now this check
can be false for the packet, e.g. when sending it in the two fragments,
this prevents successful PMTU updates in such case. The next attempts
to send the packet lead to the same tx error. Moreover, vti6 initial
MTU value relies on PMTU adjustments.

This issue can be reproduced with the following LTP test script:
    udp_ipsec_vti.sh -6 -p ah -m tunnel -s 2000

Fixes: ccd740cb ("vti6: Add pmtu handling to vti6_xmit.")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>