Recheck curthread->td_su after the VFS_SYNC() call, and re-sync if the
ast was rescheduled during VFS_SYNC(). It is possible that enough
parallel writes or slow/hung volume result in VFS_SYNC() deferring to
the ast flushing of workqueue.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week