netfilter: nfnetlink_queue: use y2038 safe timestamp

The __build_packet_message function fills a nfulnl_msg_packet_timestamp
structure that uses 64-bit seconds and is therefore y2038 safe, but
it uses an intermediate 'struct timespec' which is not.

This trivially changes the code to use 'struct timespec64' instead,
to correct the result on 32-bit architectures.

This is a copy and paste of Arnd's original patch for nfnetlink_log.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>