ext4: use EXT_MAX_BLOCKS in ext4_es_can_be_merged()

In ext4_es_can_be_merged() when checking whether we can merge two
extents we should use EXT_MAX_BLOCKS instead of defining it manually.
Also if it is really the case we should notify userspace because clearly
there is a bug in extent status tree implementation since this should
never happen.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

fs/ext4/extents_status.c

@@ -344,8 +344,14 @@ static int ext4_es_can_be_merged(struct extent_status *es1,
 	if (ext4_es_status(es1) != ext4_es_status(es2))
 		return 0;
 
-	if (((__u64) es1->es_len) + es2->es_len > 0xFFFFFFFFULL)
+	if (((__u64) es1->es_len) + es2->es_len > EXT_MAX_BLOCKS) {
+		pr_warn("ES assertion failed when merging extents. "
+			"The sum of lengths of es1 (%d) and es2 (%d) "
+			"is bigger than allowed file size (%d)\n",
+			es1->es_len, es2->es_len, EXT_MAX_BLOCKS);
+		WARN_ON(1);
 		return 0;
+	}
 
 	if (((__u64) es1->es_lblk) + es1->es_len != es2->es_lblk)
 		return 0;