netfilter: nft_dynset: fix element timeout for HZ != 1000

netfilter: nft_dynset: fix element timeout for HZ != 1000

With HZ=100 element timeout in dynamic sets (i.e. flow tables) is 10 times
higher than configured.

Add proper conversion to/from jiffies, when interacting with userspace.

I tested this on Linux 4.8.1, and it applies cleanly to current nf and
nf-next trees.

Fixes: e907265f4d6d ("netfilter: nf_tables: add support for dynamic set updates")
Signed-off-by: Anders K. Pedersen <akp@cohaesio.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>