nvmet: Implement arbitration feature support

NVMe base specification v2.1 mandates support for the arbitration
feature (NVME_FEAT_ARBITRATION). Introduce the data structure
struct nvmet_feat_arbitration to define the high, medium and low
priority weight fields and the arbitration burst field of this feature
and implement the functions nvmet_get_feat_arbitration() and
nvmet_set_feat_arbitration() functions to get and set these fields.

Since there is no generic way to implement support for the arbitration
feature, these functions respectively use the controller get_feature()
and set_feature() operations to process the feature with the help of
the controller driver. If the controller driver does not implement these
operations and a get feature command or a set feature command for this
feature is received, the command is failed with an invalid field error.

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>

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