scripts/gdb: fix lx-mounts command error

scripts/gdb: fix lx-mounts command error

(gdb) lx-mounts
      mount          super_block     devname pathname fstype options
Python Exception <class 'gdb.error'>: There is no member named list.
Error occurred in Python: There is no member named list.

We encounter the above issue after commit 2eea9ce4310d ("mounts: keep
list of mounts in an rbtree"). The commit move a mount from list into
rbtree.

So we can instead use rbtree to iterate all mounts information.

Link: https://lkml.kernel.org/r/20240723064902.124154-4-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>