libzpool: Don't call pthread_attr_setguardsize() on CHERI platforms

The kernel doesn't provide guard pages and libthr returns EINVAL if the
specified guard size is non-zero, so just avoid calling it with the aim of
making zdb usable.

lib/libzpool/kernel.c

@@ -133,7 +133,9 @@ zk_thread_create(const char *name, void (*func)(void *), void *arg,
 	 * multiple of system page size.
 	 */
 	VERIFY0(pthread_attr_setstacksize(&attr, stksize));
+#ifndef __CHERI_PURE_CAPABILITY__
 	VERIFY0(pthread_attr_setguardsize(&attr, PAGESIZE));
+#endif
 
 	VERIFY(ztw = malloc(sizeof (*ztw)));
 	ztw->func = func;