exit: Implement kthread_exit

The way the per task_struct exit_code is used by kernel threads is not
quite compatible how it is used by userspace applications.  The low
byte of the userspace exit_code value encodes the exit signal.  While
kthreads just use the value as an int holding ordinary kernel function
exit status like -EPERM.

Add kthread_exit to clearly separate the two kinds of uses.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e6ab568..90108fe 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -168,6 +168,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"panic",
 		"do_exit",
 		"do_task_dead",
+		"kthread_exit",
 		"make_task_dead",
 		"__module_put_and_exit",
 		"complete_and_exit",