wait: allow sys_waitid() to accept __WNOTHREAD/__WCLONE/__WALL

I see no reason why waitid() can't support other linux-specific flags
allowed in sys_wait4().

In particular this change can help if we reconsider the previous change
("wait/ptrace: assume __WALL if the child is traced") which adds the
"automagical" __WALL for debugger.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: Pedro Alves <palves@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1535,7 +1535,8 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *,
 	enum pid_type type;
 	long ret;
 
-	if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED))
+	if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED|
+			__WNOTHREAD|__WCLONE|__WALL))
 		return -EINVAL;
 	if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
 		return -EINVAL;