nvmet: add helper to revalidate bdev and file ns

This patch adds a wrapper helper to indicate size change in the bdev &
file-backed namespace when revalidating ns. This helper is needed in
order to minimize code repetition in the next patch for configfs.c and
existing admin-cmd.c.  

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