netfilter: nf_tables: Speed up selective rule dumps

If just a table name was given, nf_tables_dump_rules() continued over
the list of tables even after a match was found. The simple fix is to
exit the loop if it reached the bottom and ctx->table was not NULL.

When iterating over the table's chains, the same problem as above
existed. But worse than that, if a chain name was given the hash table
wasn't used to find the corresponding chain. Fix this by introducing a
helper function iterating over a chain's rules (and taking care of the
cb->args handling), then introduce a shortcut to it if a chain name was
given.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>