Revert "net/ipv6: fix bool/int mismatch for skip_notify_on_dev_down"

This reverts commit 9b001a7d which is
commit edf2e1d2 upstream.

It breaks the Android kernel abi, so revert it for now.  If it needs to
come back it can be done so in an abi-safe way.

Bug: 161946584
Change-Id: Ibbe04b272b916b4bc5f8ef681520c7cae4fa87a9
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>