lib/interval_tree: fix the comment of interval_tree_span_iter_next_gap()

The comment of interval_tree_span_iter_next_gap() is not exact, nodes[1]
is not always !NULL.

There are threes cases here. If there is an interior hole, the statement
is correct. If there is a tailing hole or the contiguous used range span
to the end, nodes[1] is NULL.

Link: https://lkml.kernel.org/r/20250310074938.26756-8-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michel Lespinasse <michel@lespinasse.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: RichardWeiYang

lib/interval_tree.c

@@ -20,9 +20,15 @@ EXPORT_SYMBOL_GPL(interval_tree_iter_next);
 /*
  * Roll nodes[1] into nodes[0] by advancing nodes[1] to the end of a contiguous
  * span of nodes. This makes nodes[0]->last the end of that contiguous used span
- * indexes that started at the original nodes[1]->start. nodes[1] is now the
- * first node starting the next used span. A hole span is between nodes[0]->last
- * and nodes[1]->start. nodes[1] must be !NULL.
+ * of indexes that started at the original nodes[1]->start.
+ *
+ * If there is an interior hole, nodes[1] is now the first node starting the
+ * next used span. A hole span is between nodes[0]->last and nodes[1]->start.
+ *
+ * If there is a tailing hole, nodes[1] is now NULL. A hole span is between
+ * nodes[0]->last and last_index.
+ *
+ * If the contiguous used range span to last_index, nodes[1] is set to NULL.
  */
 static void
 interval_tree_span_iter_next_gap(struct interval_tree_span_iter *state)