Revert "ipv6: save dontfrag in cork"

This reverts commit 2b572c40 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: I9db9a5fb35766419a6f57f7602e63fa9090fef2c
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>