ns: add NS_COMMON_INIT()
Add an initializer that can be used for the ns common initialization for
static namespace such as most init namespaces.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/87ecqhy2y5.ffs@tglx
Signed-off-by: Christian Brauner <brauner@kernel.org>