nft: cache: Simplify rule and set fetchers

nft: cache: Simplify rule and set fetchers

Since no incremental cache fetching happens anymore, code fetching rules
for chains or elements for sets may safely assume that whatever is in
cache also didn't get populated with rules or elements before.

Therefore no (optional) chain name needs to be passed on to
fetch_rule_cache() and fetch_set_cache() doesn't have to select for
which sets in a table to call set_fetch_elem_cb().

Signed-off-by: Phil Sutter <phil@nwl.cc>