netfilter: nft_set_hash: try later when GC hits EAGAIN on iteration

commit b079155faae94e9b3ab9337e82100a914ebb4e8d upstream.

Skip GC run if iterator rewinds to the beginning with EAGAIN, otherwise GC
might collect the same element more than once.

Fixes: f6c383b8c31a ("netfilter: nf_tables: adapt set backend to use GC transaction API")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>