scripts/gdb/stackdepot: rename pool_index to pools_num

After stackdepot evicting support patchset[1], we rename pool_index to
pools_num.

To avoid from the below issue, we rename consistently in
gdb scripts.

Python Exception <class 'gdb.error'>: No symbol "pool_index" in current
context.
Error occurred in Python: No symbol "pool_index" in current context.

[1] https://lore.kernel.org/linux-mm/cover.1700502145.git.andreyknvl@google.com/
Link: https://lkml.kernel.org/r/20231129065142.13375-3-Kuan-Ying.Lee@mediatek.com
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Qun-Wei Lin <qun-wei.lin@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>