netdev-linux: Skip miimon execution when disabled

netdev-linux: Skip miimon execution when disabled

When dealing with a large number of ports, one of the performance
bottlenecks is that we loop through all netdevs in the main loop. Miimon
is a contributor to this, checking all devices even if it has never been
enabled.

This patch introduces a counter for the number of netdevs with miimon
configured. If this is 0, then we skip miimon_run() and miimon_wait().
In a test environment of 5000 internal ports and 50 tunnel ports with
bfd, this reduces CPU usage from about 50% to about 45%.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>