watchdog/wdt-main: Use cpumask_of() to avoid cpumask var on stack

In general it's preferable to avoid placing cpumasks on the stack, as
for large values of NR_CPUS these can consume significant amounts of
stack space and make stack overflows more likely.

Use cpumask_of() to avoid the need for a temporary cpumask on the stack

Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/TYTP286MB3564B037A81DAAE1AC3A16F3CA062@TYTP286MB3564.JPNP286.PROD.OUTLOOK.COM


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>