IB/opa_vnic: Use GFP_ATOMIC while sending trap

Pass GFP_ATOMIC flag to ib_create_send_mad() while sending trap as it
can be triggered from the atomic context.

Fix the following trace with debug kernel.

BUG: sleeping function called from invalid context at mm/slab.h:432
in_atomic(): 1, irqs_disabled(): 0, pid: 1771, name: NetworkManager
Call Trace:
 dump_stack+0x63/0x90
 ___might_sleep+0xda/0x130
 __might_sleep+0x4a/0x90
 __kmalloc+0x19e/0x220
 ? ib_create_send_mad+0xea/0x390 [ib_core]
 ib_create_send_mad+0xea/0x390 [ib_core]
 opa_vnic_vema_send_trap+0x17b/0x460 [opa_vnic]
 opa_vnic_vema_report_event+0x57/0x80 [opa_vnic]
 opa_vnic_mac_send_event+0xaa/0xf0 [opa_vnic]
 opa_vnic_set_rx_mode+0x17/0x30 [opa_vnic]
 __dev_set_rx_mode+0x52/0x90
 dev_set_rx_mode+0x26/0x40
 __dev_open+0xe8/0x140

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>