netfilter: nf_tables: don't skip expired elements during walk

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables: don't skip expired elements during walk

There is an asymmetry between commit/abort and preparation phase if the

following conditions are met:

1. set is a verdict map ("1.2.3.4 : jump foo")

2. timeouts are enabled

In this case, following sequence is problematic:

1. element E in set S refers to chain C

2. userspace requests removal of set S

3. kernel does a set walk to decrement chain->use count for all elements

from preparation phase

4. kernel does another set walk to remove elements from the commit phase

(or another walk to do a chain->use increment for all elements from

abort phase)

If E has already expired in 1), it will be ignored during list walk, so its use count

won't have been changed.

Then, when set is culled, ->destroy callback will zap the element via

nf_tables_set_elem_destroy(), but this function is only safe for

elements that have been deactivated earlier from the preparation phase:

lack of earlier deactivate removes the element but leaks the chain use

count, which results in a WARN splat when the chain gets removed later,

plus a leak of the nft_chain structure.

Update pipapo_get() not to skip expired elements, otherwise flush

command reports bogus ENOENT errors.