xen/pvcalls: use WARN_ON(1) instead of __WARN()

__WARN() is an internal helper that is only available on
some architectures, but causes a build error e.g. on ARM64
in some configurations:

drivers/xen/pvcalls-back.c: In function 'set_backend_state':
drivers/xen/pvcalls-back.c:1097:5: error: implicit declaration of function '__WARN' [-Werror=implicit-function-declaration]

Unfortunately, there is no equivalent of BUG() that takes no
arguments, but WARN_ON(1) is commonly used in other drivers
and works on all configurations.

Fixes: 7160378206b2 ("xen/pvcalls: xenbus state handling")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>