mm/vmalloc: fix comments about vmap_area struct

The vmap_area_root should be in the "busy" tree and the
free_vmap_area_root should be in the "free" tree.

Link: https://lkml.kernel.org/r/20220305011510.33596-1-libang.linuxer@gmail.com
Fixes: 688fcbfc ("mm/vmalloc: modify struct vmap_area to reduce its size")
Signed-off-by: Bang Li <libang.linuxer@gmail.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Pengfei Li <lpf.vector@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/vmalloc.h

@@ -80,8 +80,8 @@ struct vmap_area {
 	/*
 	 * The following two variables can be packed, because
 	 * a vmap_area object can be either:
-	 *    1) in "free" tree (root is vmap_area_root)
-	 *    2) or "busy" tree (root is free_vmap_area_root)
+	 *    1) in "free" tree (root is free_vmap_area_root)
+	 *    2) or "busy" tree (root is vmap_area_root)
 	 */
 	union {
 		unsigned long subtree_max_size; /* in "free" tree */