lib/stackdepot: Fix outdated comments

Replace "depot_save_stack" with "stack_depot_save" in code comments because
depot_save_stack() was replaced in commit c0cfc337 ("lib/stackdepot:
Provide functions which operate on plain storage arrays") and removed in
commit 56d8f079 ("lib/stackdepot: Remove obsolete functions")

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190815113246.18478-1-miles.chen@mediatek.com