net: ptp: mark filter as __initdata

sk_unattached_filter_create() will copy the filter's instructions so we
don't need to have the master copy hanging around after initialization.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>