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/[email protected]
Signed-off-by: Colin Ian King <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Colin Ian King

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);