nvmet: revalidate-ns & generate AEN from configfs

Add a new attribute "revalidate_size" for the namespace which allows
user to revalidate and generate the AEN if needed. This attribute is
needed so that we can install userspace rules with systemd service based
on inotify/fsnotify/uevent. The registered callback for such a service
will end up writing to this attribute to generate AEN if needed.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimbeg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>