selftests: add a test for the foreign mnt ns extensions

This tests both statmount and listmount to make sure they work with the
extensions that allow us to specify a mount ns to enter in order to find
the mount entries.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/2d1a35bc9ab94b4656c056c420f25e429e7eb0b1.1719243756.git.josef@toxicpanda.com


Signed-off-by: Christian Brauner <brauner@kernel.org>