Documentation/watchdog: check return value for magic close

A recent commit added a write to the watchdog test code for doing the "magic
close", but that caused a compile-time warning:

Documentation/watchdog/src/watchdog-test.c: In function ‘main’:
Documentation/watchdog/src/watchdog-test.c:94:5: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]

This changes the code to print a runtime warning if the write fails.

Fixes: 5a2d3de1 ("Documentation/watchdog: add support for magic close to watchdog-test")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>