nvmet: Implement interrupt config feature support

The NVMe base specifications v2.1 mandate supporting the interrupt
config feature (NVME_FEAT_IRQ_CONFIG) for PCI controllers. Introduce the
data structure struct nvmet_feat_irq_config to define the coalescing
disabled (cd) and interrupt vector (iv) fields of this feature and
implement the functions nvmet_get_feat_irq_config() and
nvmet_set_feat_irq_config() functions to get and set these fields. These
functions respectively use the controller get_feature() and
set_feature() operations to fill and handle the fields of struct
nvmet_feat_irq_config.

Support for this feature is prohibited for fabrics controllers. If a get
feature command or a set feature command for this feature is received
for a fabrics controller, 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>