ibmvnic: queue reset work in system_long_wq

The reset process for ibmvnic commonly takes multiple seconds, clearly
making it inappropriate for schedule_work/system_wq. The reason to make
this change is that ibmvnic's use of the default system-wide workqueue
for a relatively long-running work item can negatively affect other
workqueue users. So, queue the relatively slow reset job to the
system_long_wq.

Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Lijun Pan <lijunp213@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>