sparc: Use GFP_ATOMIC in ldc_alloc_exp_dring() as it can be called in softirq context

Since it is possible for vnet_event_napi to end up doing
vnet_control_pkt_engine -> ... -> vnet_send_attr ->
vnet_port_alloc_tx_ring -> ldc_alloc_exp_dring -> kzalloc()
(i.e., in softirq context), kzalloc() should be called with
GFP_ATOMIC from ldc_alloc_exp_dring.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>