pool: Check early that we can sleep

Check early that we actuallly are in a sleepable context in pool_get().
There is an equivalent check in malloc().

from jsg@, ok jca@

Author: cludwig

sys/kern/subr_pool.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: subr_pool.c,v 1.241 2025/07/21 20:36:41 bluhm Exp $	*/
+/*	$OpenBSD: subr_pool.c,v 1.242 2025/08/01 19:00:38 cludwig Exp $	*/
 /*	$NetBSD: subr_pool.c,v 1.61 2001/09/26 07:14:56 chs Exp $	*/
 
 /*-
@@ -562,6 +562,9 @@ pool_get(struct pool *pp, int flags)
 	void *v = NULL;
 	int slowdown = 0;
 
+	if (flags & PR_WAITOK)
+		assertwaitok();
+
 	KASSERT(flags & (PR_WAITOK | PR_NOWAIT));
 	if (pp->pr_flags & PR_RWLOCK)
 		KASSERT(flags & PR_WAITOK);