nvmet: Introduce get/set_feature controller operations

The implementation of some features cannot always be done generically by
the target core code. Arbitraion and IRQ coalescing features are
examples of such features: their implementation must be provided (at
least partially) by the target controller driver.

Introduce the set_feature() and get_feature() controller fabrics
operations (in struct nvmet_fabrics_ops) to allow supporting such
features.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>