selftests: add binderfs selftests

selftests: add binderfs selftests

This adds the promised selftest for binderfs. It will verify the following
things:
- binderfs mounting works
- binder device allocation works
- performing a binder ioctl() request through a binderfs device works
- binder device removal works
- binder-control removal fails
- binderfs unmounting works

The tests are performed both privileged and unprivileged. The latter
verifies that binderfs behaves correctly in user namespaces.

Cc: Todd Kjos <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Acked-by: Shuah Khan <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>