lib/stackdepot: export save/fetch stack for drivers

Some drivers would like to record stacktraces in order to aide leak
tracing.  As stackdepot already provides a facility for only storing the
unique traces, thereby reducing the memory required, export that
functionality for use by drivers.

The code was originally created for KASAN and moved under lib in commit
cd11016 ("mm, kasan: stackdepot implementation.  Enable stackdepot
for SLAB") so that it could be shared with mm/.  In turn, we want to
share it now with drivers.

Link: http://lkml.kernel.org/r/20161108133209.22704-1-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/stackdepot.c

@@ -192,6 +192,7 @@ void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
 	trace->entries = stack->entries;
 	trace->skip = 0;
 }
+EXPORT_SYMBOL_GPL(depot_fetch_stack);
 
 /**
  * depot_save_stack - save stack in a stack depot.
@@ -283,3 +284,4 @@ depot_stack_handle_t depot_save_stack(struct stack_trace *trace,
 fast_exit:
 	return retval;
 }
+EXPORT_SYMBOL_GPL(depot_save_stack);