l2arc space accounting mismatch

Use write_psize instead of write_asize when doing vdev_space_update.
Without this change the accounting of L2ARC usage would be wrong and
give 16EB free space because the number became negative and overflows.

Obtained from:	FreeNAS (issue openzfs#6239)
MFC after:	2 weeks

fixes ZFS on Linux:

openzfs#3114
openzfs#3400

module/zfs/arc.c

@@ -5904,7 +5904,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz,
 	ARCSTAT_INCR(arcstat_l2_write_bytes, write_asize);
 	ARCSTAT_INCR(arcstat_l2_size, write_sz);
 	ARCSTAT_INCR(arcstat_l2_asize, write_asize);
-	vdev_space_update(dev->l2ad_vdev, write_asize, 0, 0);
+	vdev_space_update(dev->l2ad_vdev, write_psize, 0, 0);
 
 	/*
 	 * Bump device hand to the device start if it is approaching the end.