selftests/fs/mount-notify: add a test variant running inside userns

unshare userns in addition to mntns and verify that:

1. watching tmpfs mounted inside userns is allowed with any mark type
2. watching orig root with filesystem mark type is not allowed
3. watching mntns of orig userns is not allowed
4. watching mntns in userns where fanotify_init was called is allowed

mount events are only tested with the last case of mntns mark.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/20250509133240.529330-9-amir73il@gmail.com
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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