bridge: rename struct bridge_mcast_query/querier

The current naming of these two structs is very random, in that
reversing their naming would not make any semantical difference.

This patch tries to make the naming less confusing by giving them a more
specific, distinguishable naming.

This is also useful for the upcoming patches reintroducing the
"struct bridge_mcast_querier" but for storing information about the
selected querier (no matter if our own or a foreign querier).

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>