nft: Fix leaks in ebt_add_policy_rule()

nft: Fix leaks in ebt_add_policy_rule()

The function leaked memory allocated in temporary struct
iptables_command_state, clean it immediately after use.

In any of the udata-related error cases, allocated nftnl_rule would
leak, fix this by introducing a common error path to goto.

In regular code path, the allocated nftnl_rule would still leak:
batch_obj_del() does not free rules in NFT_COMPAT_RULE_APPEND jobs, as
they typically sit in cache as well. Policy rules in turn weren't added
to cache: They are created immediately before commit and never
referenced from other rules. Add them now so they are freed just like
regular rules.

Fixes: aff1162b3e4b7 ("ebtables-nft: Support user-defined chain policies")
Signed-off-by: Phil Sutter <phil@nwl.cc>