bridge: Add hash elasticity/max sysfs entries

This patch allows the user to control the hash elasticity/max
parameters.  The elasticity setting does not take effect until
the next new multicast group is added.  At which point it is
checked and if after rehashing it still can't be satisfied then
snooping will be disabled.

The max setting on the other hand takes effect immediately.  It
must be a power of two and cannot be set to a value less than the
current number of multicast group entries.  This is the only way
to shrink the multicast hash.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>