net: bridge: increase multicast's default maximum number of entries

bridge's default hash_max was 512 which is rather conservative, now that
we're using the generic rhashtable API which autoshrinks let's increase
it to 4096 and move it to a define in br_private.h.

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