Revert "ipv6: save dontfrag in cork"

This reverts commit 8ebf2709 which is
commit a18dfa99 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ia31c97e7cbfef365b367ce1e3a9839ccecfd3f31
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>