dm writecache: skip writecache_wait for pmem mode

The array bio_in_progress[2] only have chance to be increased and
decreased with ssd mode. For pmem mode, they are not involved at all.
So skip writecache_wait_for_ios in writecache_flush for pmem.
Suggested-by: Doris Yu <tyu1@lenovo.com>
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -726,6 +726,7 @@ static void writecache_flush(struct dm_writecache *wc)
 	}
 	writecache_commit_flushed(wc);
 
+	if (!WC_MODE_PMEM(wc))
 		writecache_wait_for_ios(wc, WRITE);
 
 	wc->seq_count++;