netfilter: core: return EBUSY in case NAT hook is already in use

EEXIST is used for an object that already exists, with the same
name/handle. However, there no same object there, instead there is a
object that is using the single slot that is available for NAT hooks
since patch f92b40a8 ("netfilter: core: only allow one nat hook per
hook point"). Let's change this return value before this behaviour gets
exposed in the first -rc.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>