exit: Fix Sparse errors and warnings

This patch fixes the following sparse error:
kernel/exit.c:627:25: error: incompatible types in comparison expression

And the following warning:
kernel/exit.c:626:40: warning: incorrect type in assignment

Signed-off-by: Madhuparna Bhowmik <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
[[e-mail]: edit commit message]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

kernel/exit.c

···
 	reaper = find_new_reaper(father, reaper);
 	list_for_each_entry(p, &father->children, sibling) {
 		for_each_thread(p, t) {
-			t->real_parent = reaper;
-			BUG_ON((!t->ptrace) != (t->parent == father));
+			RCU_INIT_POINTER(t->real_parent, reaper);
+			BUG_ON((!t->ptrace) != (rcu_access_pointer(t->parent) == father));
 			if (likely(!t->ptrace))
 				t->parent = t->real_parent;
 			if (t->pdeath_signal)