lib/stackdepot: rename init_stack_pool

Rename init_stack_pool to depot_init_pool to align the name with
depot_alloc_stack.

No functional changes.

Link: https://lkml.kernel.org/r/23106a3e291d8df0aba33c0e2fe86dc596286479.1676063693.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>