netfilter: nft_hash: do not dump the auto generated seed

This can prevent the nft utility from printing out the auto generated
seed to the user, which is unnecessary and confusing.

Fixes: cb1b69b0 ("netfilter: nf_tables: add hash expression")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>