Revert "ipv6: save dontfrag in cork"

This reverts commit 29533d1a 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: I8f9a6b9c5c5ff2541cba90431bbb848a4ec98c10
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>