netfilter: nf_tables: add nft_set_elem_expr_destroy() and use it

This patch adds nft_set_elem_expr_destroy() to destroy stateful
expressions in set elements.

This patch also updates the commit path to call this function to invoke
expr->ops->destroy_clone when required.

This is implicitly fixing up a module reference counter leak and
a memory leak in expressions that allocated internal state, e.g.
nft_counter.

Fixes: 40944452 ("netfilter: nf_tables: add elements with stateful expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>