mm/damon/core: nullify pointer ctx->kdamond with a NULL

Currently a plain integer is being used to nullify the pointer
ctx->kdamond.  Use NULL instead.  Cleans up sparse warning:

  mm/damon/core.c:317:40: warning: Using plain integer as NULL pointer

Link: https://lkml.kernel.org/r/20210925215908.181226-1-colin.king@canonical.comSigned-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/damon/core.c

@@ -314,7 +314,7 @@ static int __damon_start(struct damon_ctx *ctx)
 				nr_running_ctxs);
 		if (IS_ERR(ctx->kdamond)) {
 			err = PTR_ERR(ctx->kdamond);
-			ctx->kdamond = 0;
+			ctx->kdamond = NULL;
 		}
 	}
 	mutex_unlock(&ctx->kdamond_lock);