dma-buf: Call rcu_read_unlock() before locking cursor->obj->rw

The function is called in between rcu_read_lock()/rcu_read_unlock().
Dropping this lock to wait for the writer thread to finish on FreeBSD
was lost in the code refactoring.

Author: dumbbell

drivers/dma-buf/dma-resv.c

@@ -369,8 +369,10 @@ static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
 	 * for write may be blocked. In that case reader thread should
 	 * be blocked too.
 	 */
+	rcu_read_unlock();
 	rw_rlock(&cursor->obj->rw);
 	rw_runlock(&cursor->obj->rw);
+	rcu_read_lock();
 #endif
 }