fix pthread_create return value with PTHREAD_EXPLICIT_SCHED

due to moved code, commit b8742f3260
inadvertently used the return value of __clone, rather than the return
value of SYS_sched_setscheduler in the new thread, to check whether it
needed to report failure. since a successful __clone returns the tid
of the new thread, which is never zero, this caused pthread_create
always to return with an invalid error number in the code path for
PTHREAD_EXPLICIT_SCHED.

this regression was not present in any releases.

src/thread/pthread_create.c

@@ -306,6 +306,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
 
 	if (do_sched) {
 		__futexwait(&ssa.futex, -1, 1);
+		ret = ssa.futex;
 		if (ret) return ret;
 	}