rtbsd: Make rtbsd 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 bsd are destroyed, the rtbsd notifier is
unregistered, possibly causing memory to be freed. This causes a
race condition with the main thread which calls rtbsd_notifier_run
periodically to check for any netdev change events.

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

Note this patch removes rtbsd_notifier_run() in
rtbsd_notifier_register() due to locking requirements. Since the
rtbsd_notifier_run() is always run by the main thread often,
receiving stale notifications from the notifier is unlikely.

Bug #1258532
Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>