4088 use after free in arc_release()

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Dan McDonald <danmcd@nexenta.com>

usr/src/uts/common/fs/zfs/arc.c

@@ -3332,6 +3332,7 @@ arc_release(arc_buf_t *buf, void *tag)
 	if (l2hdr) {
 		mutex_enter(&l2arc_buflist_mtx);
 		hdr->b_l2hdr = NULL;
+		list_remove(l2hdr->b_dev->l2ad_buflist, hdr);
 	}
 	buf_size = hdr->b_size;
 
@@ -3415,7 +3416,6 @@ arc_release(arc_buf_t *buf, void *tag)
 
 	if (l2hdr) {
 		ARCSTAT_INCR(arcstat_l2_asize, -l2hdr->b_asize);
-		list_remove(l2hdr->b_dev->l2ad_buflist, hdr);
 		kmem_free(l2hdr, sizeof (l2arc_buf_hdr_t));
 		ARCSTAT_INCR(arcstat_l2_size, -buf_size);
 		mutex_exit(&l2arc_buflist_mtx);