Revert "ipv6: save dontfrag in cork"

This reverts commit 4f809be9 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: I92d1731431ba8fdf5be8df5457bc73cd5ae4ffaf
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>