netfilter: conntrack: free extension area immediately

Instead of waiting for rcu grace period just free it directly.

This is safe because conntrack lookup doesn't consider extensions.

Other accesses happen while ct->ext can't be free'd, either because
a ct refcount was taken or because the conntrack hash bucket lock or
the dying list spinlock have been taken.

This allows to remove __krealloc in a followup patch, netfilter was the
only user.

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