mm: use do_futex() instead of sys_futex() in mm_release()

sys_futex() is a wrapper to do_futex() which does not modify any
values here:

- uaddr, val and val3 are kept the same

- op is masked with FUTEX_CMD_MASK, but is always set to FUTEX_WAKE.
Therefore, val2 is always 0.

- as utime is set to NULL, *timeout is NULL

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/[e-mail]

Cc: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Darren Hart <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Reviewed-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Dominik Brodowski <[e-mail]>

include/linux/futex.h

···
 struct mm_struct;
 struct task_struct;
 
-long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
-	      u32 __user *uaddr2, u32 val2, u32 val3);
-
 extern int
 handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi);
 
···
 
 #ifdef CONFIG_FUTEX
 extern void exit_robust_list(struct task_struct *curr);
+
+long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
+	      u32 __user *uaddr2, u32 val2, u32 val3);
 #ifdef CONFIG_HAVE_FUTEX_CMPXCHG
 #define futex_cmpxchg_enabled 1
 #else
···
 #else
 static inline void exit_robust_list(struct task_struct *curr)
 {
+}
+
+static inline long do_futex(u32 __user *uaddr, int op, u32 val,
+			    ktime_t *timeout, u32 __user *uaddr2,
+			    u32 val2, u32 val3)
+{
+	return -EINVAL;
 }
 #endif
 

kernel/fork.c

···
 			 * not set up a proper pointer then tough luck.
 			 */
 			put_user(0, tsk->clear_child_tid);
-			sys_futex(tsk->clear_child_tid, FUTEX_WAKE,
-					1, NULL, NULL, 0);
+			do_futex(tsk->clear_child_tid, FUTEX_WAKE,
+					1, NULL, NULL, 0, 0);
 		}
 		tsk->clear_child_tid = NULL;
 	}