netfilter: nft_last: honor NFTA_LAST_SET on restoration

NFTA_LAST_SET tells us if this expression has ever seen a packet, do not
ignore this attribute when restoring the ruleset.

Fixes: 836382dc ("netfilter: nf_tables: add last expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>