If the lock passed to getdirtybuf() is the softdep lock then the background write completed wakeup could be missed.  Close the race by grabbing the lock normally used for protection of bp->b_xflags.
If the lock passed to getdirtybuf() is the softdep lock then the background
write completed wakeup could be missed.  Close the race by grabbing the lock
normally used for protection of bp->b_xflags.

Reviewed by:	truckman

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