According to the NVMe specification, the over temperature threshold and
under temperature threshold features shall be implemented for Composite
Temperature if a non-zero WCTEMP field value is reported in the Identify
Controller data structure.  The features are also implemented for all
implemented temperature sensors (i.e., all Temperature Sensor fields that
report a non-zero value).

This provides the over temperature threshold and under temperature
threshold for each sensor as temperature min and max values of hwmon
sysfs attributes.

The WCTEMP is already provided as a temperature max value for Composite
Temperature, but this change isn't incompatible.  Because the default
value of the over temperature threshold for Composite Temperature is
the WCTEMP.

Now the alarm attribute for Composite Temperature indicates one of the
temperature is outside of a temperature threshold.  Because there is only
a single bit in Critical Warning field that indicates a temperature is
outside of a threshold.

Example output from the "sensors" command:

nvme-pci-0100
Adapter: PCI adapter
Composite:    +33.9°C  (low  = -273.1°C, high = +69.8°C)
                       (crit = +79.8°C)
Sensor 1:     +34.9°C  (low  = -273.1°C, high = +65261.8°C)
Sensor 2:     +31.9°C  (low  = -273.1°C, high = +65261.8°C)
Sensor 5:     +47.9°C  (low  = -273.1°C, high = +65261.8°C)

This also adds helper macros for kelvin from/to milli Celsius conversion,
and replaces the repeated code in hwmon.c.

Cc: Keith Busch <kbusch@kernel.org>
Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Jean Delvare <jdelvare@suse.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Tested-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NKeith Busch <kbusch@kernel.org>