src: make hash seed attribute optional

src: make hash seed attribute optional

The hash expression requires a seed attribute to call the jhash
operation, eg.

 # nft add rule x y meta mark set jhash ip saddr . ip daddr mod 2 \
	seed 0xdeadbeef

With this patch the seed attribute is optional and it's generated by a
random function from userspace, eg.

 # nft add rule x y meta mark set jhash ip saddr . ip daddr mod 2

The kernel will take care of generate a random seed.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>