scripts/gdb: add iteration function for rbtree

scripts/gdb: add iteration function for rbtree

Add inorder iteration function for rbtree usage.

This is a preparation patch for the next patch to fix the gdb mounts
issue.

Link: https://lkml.kernel.org/r/20240723064902.124154-3-kuan-ying.lee@canonical.com
Fixes: 2eea9ce4310d ("mounts: keep list of mounts in an rbtree")
Signed-off-by: Kuan-Ying Lee <kuan-ying.lee@canonical.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>