cache: incorrect flush flag for table/chain

cache: incorrect flush flag for table/chain

After the new cache system, nft raises a table error flushing a chain in
a transaction.

 # nft "flush chain ip nftlb filter-newfarm ; \
    add rule ip nftlb filter-newfarm update \
    @persist-newfarm {  ip saddr : ct mark } ; \
    flush chain ip nftlb nat-newfarm"
 Error: No such file or directory
 flush chain ip nftlb filter-newfarm ; add rule ip nftlb (...)
                                                   ^^^^^

This patch sets the cache flag properly to save this case.

Fixes: 01e5c6f0ed031 ("src: add cache level flags")
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>