futex: Fix kernel-doc notation warning

Fix a kernel-doc warning in kernel/futex.c by adding notation
for @ret.

../kernel/futex.c:1187: warning: Function parameter or member 'ret' not described in 'wait_for_owner_exiting'

Fixes: 3ef240ea ("futex: Prevent exit livelock")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/223be78c-f3c8-52df-836d-c5fb8e7907e9@infradead.org

kernel/futex.c

@@ -1178,6 +1178,7 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
 
 /**
  * wait_for_owner_exiting - Block until the owner has exited
+ * @ret: owner's current futex lock status
  * @exiting:	Pointer to the exiting task
  *
  * Caller must hold a refcount on @exiting.