sched/numa: Replace hard-coded number by a define in numa_task_group()

While allocating group fault stats, task_numa_group()
is using a hard coded number 4. Replace this by
NR_NUMA_HINT_FAULT_STATS.

No functionality change in this commit.

Signed-off-by: Bharata B Rao <bharata@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20211004105706.3669-2-bharata@amd.com