binder: Fix comment headers on binder_alloc_prepare_to_free()

binder_alloc_buffer_lookup() doesn't exist and is named
"binder_alloc_prepare_to_free()". Correct the code comments to reflect
this.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20190930201250.139554-1-joel@joelfernandes.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -156,7 +156,7 @@ static struct binder_buffer *binder_alloc_prepare_to_free_locked(
 }
 
 /**
- * binder_alloc_buffer_lookup() - get buffer given user ptr
+ * binder_alloc_prepare_to_free() - get buffer given user ptr
  * @alloc:	binder_alloc for this proc
  * @user_ptr:	User pointer to buffer data
  *