IB/ehca: Use kthread_create_on_node()

Since create_comp_task() creates percpu kthread, it makes sense to use
kthread_create_on_node() to get proper NUMA affinity for kthread
stack.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>