netfilter: nf_tables: revert do not remove elements if set backend implements .abort

nf_tables_abort_release() path calls nft_set_elem_destroy() for
NFT_MSG_NEWSETELEM which releases the element, however, a reference to
the element still remains in the working copy.

Fixes: ebd032fa ("netfilter: nf_tables: do not remove elements if set backend implements .abort")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>