binder: simplify the return expression of binder_mmap

Simplify the return expression.

Acked-by: Martijn Coenen <maco@android.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Link: https://lore.kernel.org/r/20200929015216.1829946-1-liushixin2@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>