lib/stackdepot: rename slab to pool

lib/stackdepot: rename slab to pool

Use "pool" instead of "slab" for naming memory regions stack depot
uses to store stack traces. Using "slab" is confusing, as stack depot
pools have nothing to do with the slab allocator.

Also give better names to pool-related global variables: change
"depot_" prefix to "pool_" to point out that these variables are
related to stack depot pools.

Also rename the slabindex (poolindex) field in handle_parts to pool_index
to align its name with the pool_index global variable.

No functional changes.

Link: https://lkml.kernel.org/r/923c507edb350c3b6ef85860f36be489dfc0ad21.1676063693.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>