sysfs/cpu: Use DEVICE_ATTR_RO for nohz_full attribute

sysfs/cpu: Use DEVICE_ATTR_RO for nohz_full attribute

The /sys/devices/system/cpu/nohz_full file is a read-only attribute that
reports the CPUs configured for tickless operation (CONFIG_NO_HZ_FULL=y).

The current definition uses the generic DEVICE_ATTR macro, which
unnecessarily requires specifying the file mode (0444) and a NULL
store operation pointer.

This patch converts the definition to use the dedicated DEVICE_ATTR_RO
macro. This correctly expresses the read-only nature of the attribute,
removes the redundant mode field, and simplifies the code. As a related
cleanup, rename the show function from print_cpus_nohz_full() to the
standard nohz_full_show() for consistency with common sysfs attribute
naming conventions.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Link: https://patch.msgid.link/20251011011830.6670-2-atomlin@atomlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>