Merge branch 'ip6-tunnel-tx-fixes'

Tom Herbert says:

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ip6: Transmit tunneling fixes

Several fixes suggested by Alexander.

Tested: Running netperf TCP_STREAM with gretap and keyid configured.
Visually verified that MTU is correctly being set. Did not test HW
offload (Alexander plese try)
====================

Tested-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>