lib/stackdepot: use pr_fmt to define message format

Use pr_fmt to define the format for printing stack depot messages instead
of duplicating the "Stack Depot" prefix in each message.

Link: https://lkml.kernel.org/r/3d09db0171a0e92ff3eb0ee74de74558bc9b56c4.1676063693.git.andreyknvl@google.com


Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>