xen/unpopulated-alloc: fix error return code in fill_list()

Fix to return a negative error code from the error handling case instead
of 0, as done elsewhere in this function.

Fixes: a4574f63edc6 ("mm/memremap_pages: convert to 'struct range'")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Zhen Lei <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/unpopulated-alloc.c

···
 	}
 
 	pgmap = kzalloc(sizeof(*pgmap), GFP_KERNEL);
-	if (!pgmap)
+	if (!pgmap) {
+		ret = -ENOMEM;
 		goto err_pgmap;
+	}
 
 	pgmap->type = MEMORY_DEVICE_GENERIC;
 	pgmap->range = (struct range) {