netfilter: nft_meta: use 64-bit time arithmetic

On 32-bit architectures, get_seconds() returns an unsigned 32-bit
time value, which also matches the type used in the nft_meta
code. This will not overflow in year 2038 as a time_t would, but
it still suffers from the overflow problem later on in year 2106.

Change this instance to use the time64_t type consistently
and avoid the deprecated get_seconds().

The nft_meta_weekday() calculation potentially gets a little slower
on 32-bit architectures, but now it has the same behavior as on
64-bit architectures and does not overflow.

Fixes: 63d10e12 ("netfilter: nft_meta: support for time matching")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>