binderfs: handle !CONFIG_IPC_NS builds

kbuild reported a build faile in [1]. This is triggered when CONFIG_IPC_NS
is not set. So let's make the use of init_ipc_ns conditional on
CONFIG_IPC_NS being set.

[1]: https://lists.01.org/pipermail/kbuild-all/2019-January/056903.html



Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 7fefaadd6a962987baac50e7b3c4c3d5ef9b55c6)