route-table: Make route-table module thread-safe.

Due to patch fe83f8 (netdev: Remove netdev from global shash when
the user is changing interface configuration), netdevs can be
destructed and deallocated by revalidator and RCU threads. When
netdevs with class vport are destroyed, the routing table is
unregistered, possibly causing memory to be freed. This causes a
race condition with the main thread which calls route_table_run
periodically to check for routing table updates.

This patch makes the route-table module thread-safe via a mutex.

Bug #1258532
Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>