mm: compaction: avoid possible NULL pointer dereference in kcompactd_cpu_online

It's possible that kcompactd_run could fail to run kcompactd for a hot
added node and leave pgdat->kcompactd as NULL.  So pgdat->kcompactd should
be checked here to avoid possible NULL pointer dereference.

Link: https://lkml.kernel.org/r/20220418141253.24298-10-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Charan Teja Kalla <charante@codeaurora.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Pintu Kumar <pintu@codeaurora.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>