cache: filter out rules by chain

cache: filter out rules by chain

With an autogenerated ruleset with ~20k chains.

 # time nft list ruleset &> /dev/null

 real    0m1,712s
 user    0m1,258s
 sys     0m0,454s

Speed up listing of a specific chain:

 # time nft list chain nat MWDG-UGR-234PNG3YBUOTS5QD &> /dev/null

 real    0m0,542s
 user    0m0,251s
 sys     0m0,292s

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