netmap: add a tunable for the maximum number of VALE switches

The new dev.netmap.max_bridges sysctl tunable can be set in
loader.conf(5) to change the default maximum number of VALE
switches that can be created. Current defaults is 8.

MFC after: 2 weeks

(cherry picked from commit dd6ab49a9aebb5fbb85973d9a859d24daa5e2165)