kmemleak: add checksum to backtrace report

Change /sys/kernel/debug/kmemleak report format slightly, adding
"(extra info)" to the backtrace header:

from: "  backtrace:"
to:   "  backtrace (crc <cksum>):"

The <cksum> allows a user to see recurring backtraces without
detailed/careful reading of multiline stacks.  So after cycling
kmemleak-test a few times, I know some leaks are repeating.

  bash-5.2# grep backtrace /sys/kernel/debug/kmemleak | wc
     62     186    1792
  bash-5.2# grep backtrace /sys/kernel/debug/kmemleak | sort -u | wc
     37     111    1067

syzkaller parses kmemleak for "unreferenced object" only, so is
unaffected by this change.  Other github repos are moribund.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jim Cromie <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jimc

mm/kmemleak.c

@@ -362,7 +362,7 @@ static void print_unreferenced(struct seq_file *seq,
 	warn_or_seq_printf(seq, "  comm \"%s\", pid %d, jiffies %lu\n",
 			   object->comm, object->pid, object->jiffies);
 	hex_dump_object(seq, object);
-	warn_or_seq_printf(seq, "  backtrace:\n");
+	warn_or_seq_printf(seq, "  backtrace (crc %x):\n", object->checksum);
 
 	for (i = 0; i < nr_entries; i++) {
 		void *ptr = (void *)entries[i];