hash: generate a random seed if seed option is empty

hash: generate a random seed if seed option is empty

Typing the "nft add rule x y ct mark set jhash ip saddr mod 2" will
not generate a random seed, instead, the seed will always be zero.

So if seed option is empty, we shoulde not set the NFTA_HASH_SEED
attribute, then a random seed will be generated in the kernel.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>