netfilter: nft_set_pipapo: stop GC iteration if GC transaction allocation fails

nft_trans_gc_queue_sync() enqueues the GC transaction and it allocates a
new one. If this allocation fails, then stop this GC sync run and retry
later.

Fixes: 5f68718b ("netfilter: nf_tables: GC transaction API to avoid race with control plane")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>