Documentation/watchdog: close the fd when cmdline arg given

in the watchdog test code, the ioctl is performed on the watchdog device
and just doing exit(0) so we leak a filedescripor.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>