vdev_file_io_start() to use taskq_dispatch(TQ_PUSHPAGE)

The vdev_file_io_start() function may be processing a zio that the
txg_sync thread is waiting on.  In this case it is not safe to perform
memory allocations that may generate new I/O since this could cause a
deadlock.  To avoid this, call taskq_dispatch() with TQ_PUSHPAGE
instead of TQ_SLEEP.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#1928

module/zfs/vdev_file.c

@@ -185,7 +185,7 @@ vdev_file_io_start(zio_t *zio)
 	}
 
 	VERIFY3U(taskq_dispatch(system_taskq, vdev_file_io_strategy, zio,
-	    TQ_SLEEP), !=, 0);
+	    TQ_PUSHPAGE), !=, 0);
 
 	return (ZIO_PIPELINE_STOP);
 }