slub: STACKDEPOT: rename save_stack_trace()

slub: STACKDEPOT: rename save_stack_trace()

save_stack_trace() already exists, so change the one in
CONFIG_STACKDEPOT to be save_stack_depot_trace().

Fixes this build error:

../mm/slub.c:607:29: error: conflicting types for `save_stack_trace'
 static depot_stack_handle_t save_stack_trace(gfp_t flags)
                             ^~~~~~~~~~~~~~~~
In file included from ../include/linux/page_ext.h:6:0,
                 from ../include/linux/mm.h:25,
                 from ../mm/slub.c:13:
../include/linux/stacktrace.h:86:13: note: previous declaration of `save_stack_trace' was here
 extern void save_stack_trace(struct stack_trace *trace);
             ^~~~~~~~~~~~~~~~

from this patch in mmotm:
  Subject: mm/slub: use stackdepot to save stack trace in objects

Link: https://lkml.kernel.org/r/20210513051920.29320-1-rdunlap@infradead.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Oliver Glitta <glittao@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>