libata: Introduce ncq_prio_supported sysfs sttribute

libata: Introduce ncq_prio_supported sysfs sttribute

Currently, the only way a user can determine if a SATA device supports
NCQ priority is to try to enable the use of this feature using the
ncq_prio_enable sysfs device attribute. If enabling the feature fails,
it is because the device does not support NCQ priority. Otherwise, the
feature is enabled and success indicates that the device supports NCQ
priority.

Improve this odd interface by introducing the read-only
ncq_prio_supported sysfs device attribute to indicate if a SATA device
supports NCQ priority. The value of this attribute reflects the status
of device flag ATA_DFLAG_NCQ_PRIO, which is set only for devices
supporting NCQ priority.

Add this new sysfs attribute to the device attributes group of libahci
and libata-sata.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210816014456.2191776-10-damien.lemoal@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>