sched/fair: Fix open-coded numa_nearest_node()

task_numa_placement() searches for a nearest node to migrate by calling
for_each_node_state(). Now that we have numa_nearest_node(), switch to
using it.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Link: https://lore.kernel.org/r/20230819141239.287290-3-yury.norov@gmail.com