net: bridge: mark hash_elasticity as obsolete

Now that the bridge multicast uses the generic rhashtable interface we
can drop the hash_elasticity option as that is already done for us and
it's hardcoded to a maximum of RHT_ELASTICITY (16 currently). Add a
warning about the obsolete option when the hash_elasticity is set.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>