nvmet: make ctrl model configurable

This patch adds a new target subsys attribute which allows user to
optionally specify model name which then used in the
nvmet_execute_identify_ctrl() to fill up the nvme_id_ctrl structure.

The default value for the model is set to "Linux" for backward
compatibility.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Mark Ruijter <MRuijter@onestopsystems.com>
[chaitanya.kulkarni@wdc.com
 *Use macro for default model, coding style fixes.
 *Use RCU for accessing model in for configfs and in
  nvmet_execute_identify_ctrl().
]
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>