Revert "gtp: Use for_each_netdev_rcu() in gtp_genl_dump_pdp()."

This reverts commit 574c3f3f which is
commit 46841c70 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: Ic4e353bd2ab45d5d9f45558e031d219d1cbba39e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>