netfilter: conntrack: avoid excess memory allocation

This is now a fixed-size extension, so we don't need to pass a variable
alloc size.  This (harmless) error results in allocating 32 instead of
the needed 16 bytes for this extension as the size gets passed twice.

Fixes: 23014011 ("netfilter: conntrack: support a fixed size of 128 distinct labels")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>