fs/buffer: Convert to hotplug state machine

Install the callbacks via the state machine.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-fsdevel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20161103145021.28528-2-bigeasy@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>