scsi: block: Improve ioprio value validity checks

The introduction of the macro IOPRIO_PRIO_LEVEL() in commit eca20409
("scsi: block: ioprio: Clean up interface definition") results in an
iopriority level to always be masked using the macro IOPRIO_LEVEL_MASK, and
thus to the kernel always seeing an acceptable value for an I/O priority
level when checked in ioprio_check_cap().  Before this patch, this function
would return an error for some (but not all) invalid values for a level
valid range of [0..7].

Restore and improve the detection of invalid priority levels by introducing
the inline function ioprio_value() to check an ioprio class, level and hint
value before combining these fields into a single value to be used with
ioprio_set() or AIOs. If an invalid value for the class, level or hint of
an ioprio is detected, ioprio_value() returns an ioprio using the class
IOPRIO_CLASS_INVALID, indicating an invalid value and causing
ioprio_check_cap() to return -EINVAL.

Fixes: 6c913257 ("scsi: block: I...