dma-buf: clarify locking documentation for reservation_object_get_excl

The documentation was misleading, as for a lot of use-cases holding
the RCU read side lock is sufficient.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111165302.25556-2-l.stach@pengutronix.de

include/linux/reservation.h

@@ -228,7 +228,8 @@ reservation_object_unlock(struct reservation_object *obj)
  * @obj: the reservation object
  *
  * Returns the exclusive fence (if any).  Does NOT take a
- * reference.  The obj->lock must be held.
+ * reference. Writers must hold obj->lock, readers may only
+ * hold a RCU read side lock.
  *
  * RETURNS
  * The exclusive fence or NULL