watchdog:use 'V' to stop watchdog

[anjiahao1]

Summary

add a way to stop watchdog, follow Linux watchdog behavior:
https://www.kernel.org/doc/Documentation/watchdog/watchdog-api.txt:
Magic Close feature:

If a driver supports "Magic Close", the driver will not disable the
watchdog unless a specific magic character 'V' has been sent to
/dev/watchdog just before closing the file. If the userspace daemon
closes the file without sending this special character, the driver
will assume that the daemon (and userspace in general) died, and will
stop pinging the watchdog without disabling it first. This will then
cause a reboot if the watchdog is not re-opened in sufficient time.

Impact

watchdog driver

Testing

vela

[hartmannathan]

I wonder if this shouldn't be an optional feature (with Kconfig). Rationale: Perhaps the application developer wants to ensure that a watchdog will not be stopped accidentally due to an errant 'V' character sent to /dev/watchdog by mistake.

Suggested Kconfig name: CONFIG_WATCHDOG_MAGIC_V.

This option could be enabled (=y) by default for simplicity, while being available for users who want to harden their config.