xen-netfront: do not cast grant table reference to signed short

xen-netfront: do not cast grant table reference to signed short

While grant reference is of type uint32_t, xen-netfront erroneously casts
it to signed short in BUG_ON().

This would lead to the xen domU panic during boot-up or migration when it
is attached with lots of paravirtual devices.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Oracle-Bug: 25138362
upstream commit: 87557efc27f6a50140fb20df06a917f368ce3c66
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed by: Jack F. Vogel <jack.vogel@oracle.com>
Acked-by: Joe Jin <joe.jin@oracle.com>