netfilter: connlabels: change nf_connlabels_get bit arg to 'highest used'

nf_connlabel_set() takes the bit number that we would like to set.
nf_connlabels_get() however took the number of bits that we want to
support.

So e.g. nf_connlabels_get(32) support bits 0 to 31, but not 32.
This changes nf_connlabels_get() to take the highest bit that we want
to set.

Callers then don't have to cope with a potential integer wrap
when using nf_connlabels_get(bit + 1) anymore.

Current callers are fine, this change is only to make folloup
nft ct label set support simpler.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>