bridge: vlan: adjust rhashtable initial size and hash locks size

As Stephen pointed out the default initial size is more than we need, so
let's start small (4 elements, thus nelem_hint = 3). Also limit the hash
locks to the number of CPUs as we don't need any write-side scaling and
this looks like the minimum.

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