xen-netfront: cast grant table reference first to type int

xen-netfront: cast grant table reference first to type int

commit 269ebce4531b8edc4224259a02143181a1c1d77c upstream.

IS_ERR_VALUE() in commit 87557efc27f6a50140fb20df06a917f368ce3c66
("xen-netfront: do not cast grant table reference to signed short") would
not return true for error code unless we cast ref first to type int.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Blake Cooper <blake.cooper@braintreepayments.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>