netfilter: nf_tables: fix memleak when more than 255 elements expired

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

netfilter: nf_tables: fix memleak when more than 255 elements expired

When more than 255 elements expired we're supposed to switch to a new gc

container structure.

This never happens: u8 type will wrap before reaching the boundary

and nft_trans_gc_space() always returns true.

This means we recycle the initial gc container structure and

lose track of the elements that came before.

While at it, don't deref 'gc' after we've passed it to call_rcu.