nvmet: Introduce common execute function for get_log_page and identify

Instead of picking the sub-command handler to execute in a nested
switch statement introduce a landing functions that calls out
to the appropriate sub-command handler.

This will allow us to have a common place in the handler to check
the transfer length in a future patch.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[split patch, update change log]
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>