netfilter: nft_set_bitmap: keep a list of dummy elements

Element comments may come without any prior set flag, so we have to keep
a list of dummy struct nft_set_ext to keep this information around. This
is only useful for set dumps to userspace. From the packet path, this
set type relies on the bitmap representation. This patch simplifies the
logic since we don't need to allocate the dummy nft_set_ext structure
anymore on the fly at the cost of increasing memory consumption because
of the list of dummy struct nft_set_ext.

Fixes: 665153ff ("netfilter: nf_tables: add bitmap set type")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>