netfilter: nft_hash: track register operations

Check if the destination register already contains the data that this
osf expression performs. Always cancel register tracking for jhash since
this requires tracking multiple source registers in case of
concatenations. Perform register tracking (without bitwise) for symhash
since input does not come from source register.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>