When mounting SU-enabled mount point, wait until the softdep_flush()
thread started and incremented the stat_flush_threads [1].

Unconditionally wakeup softdep_flush threads when needed, do not try
to check wchan, which is racy and breaks abstraction.

Reported by and discussed with: glebius, neel
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks