nvme: move sysfs code to a dedicated sysfs.c file

The core.c file became long and hard to maintain. Create a dedicated
file to centralize the sysfs functionality. This is a common practice to
separate sysfs/configfs related logic from the main driver logic .c file.
For example, in the nvmet module the configfs interface has its own
dedicated file.

This patch does not include any functional changes.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
[merged dhchap memleak fixes, include nvme-auth.h]
Signed-off-by: Keith Busch <kbusch@kernel.org>