L2ARC: Move different stats updates earlier

..., before we make the header or the log block visible to others.
It should fix assertion on allocated space going negative if the
header is freed once the lock is dropped, while the write is still
going.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Fixes openzfs#16040

module/zfs/arc.c

@@ -9287,6 +9287,14 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 			hdr->b_l2hdr.b_hits = 0;
 			hdr->b_l2hdr.b_arcs_state =
 			    hdr->b_l1hdr.b_state->arcs_state;
+			arc_hdr_set_flags(hdr, ARC_FLAG_HAS_L2HDR |
+			    ARC_FLAG_L2_WRITING);
+
+			(void) zfs_refcount_add_many(&dev->l2ad_alloc,
+			    arc_hdr_size(hdr), hdr);
+			l2arc_hdr_arcstats_increment(hdr);
+			vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+
 			mutex_enter(&dev->l2ad_mtx);
 			if (pio == NULL) {
 				/*
@@ -9298,12 +9306,6 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 			}
 			list_insert_head(&dev->l2ad_buflist, hdr);
 			mutex_exit(&dev->l2ad_mtx);
-			arc_hdr_set_flags(hdr, ARC_FLAG_HAS_L2HDR |
-			    ARC_FLAG_L2_WRITING);
-
-			(void) zfs_refcount_add_many(&dev->l2ad_alloc,
-			    arc_hdr_size(hdr), hdr);
-			l2arc_hdr_arcstats_increment(hdr);
 
 			boolean_t commit = l2arc_log_blk_insert(dev, hdr);
 			mutex_exit(hash_lock);
@@ -9333,7 +9335,6 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 			write_psize += psize;
 			write_asize += asize;
 			dev->l2ad_hand += asize;
-			vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
 
 			if (commit) {
 				/* l2ad_hand will be adjusted inside. */
@@ -10585,6 +10586,8 @@ l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
 	(void) zio_nowait(wzio);
 
 	dev->l2ad_hand += asize;
+	vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
+
 	/*
 	 * Include the committed log block's pointer  in the list of pointers
 	 * to log blocks present in the L2ARC device.
@@ -10598,7 +10601,6 @@ l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
 	zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
 	zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
 	mutex_exit(&dev->l2ad_mtx);
-	vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
 
 	/* bump the kstats */
 	ARCSTAT_INCR(arcstat_l2_write_bytes, asize);