[S390] vmwatchdog: remove lock_kernel() from open() function

The lock_kernel() calls in the open() function are unnecessary because
misc_register() is called last in module_init() and concurrent open()
calls are handled by other means.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>