ext4: no need to remove extent if len is 0 in ext4_es_remove_extent()

len is 0 means no extent needs to be removed, so return immediately.
Otherwise it could trigger the following BUG_ON() in
ext4_es_remove_extent()

	end = lblk + len - 1;
	BUG_ON(end < lblk);

This could be reproduced by a simple truncate(1) command by an
unprivileged user

	truncate -s $(($((2**32 - 1)) * 4096)) /mnt/ext4/testfile

The same is true for __es_insert_extent().

Patched kernel passed xfstests regression test.
Signed-off-by: NEryu Guan <guaneryu@gmail.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: NZheng Liu <wenqing.lz@taobao.com>

fs/ext4/extents_status.c

@@ -456,6 +456,9 @@ int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
 	es_debug("add [%u/%u) %llu %llx to extent status tree of inode %lu\n",
 		 lblk, len, pblk, status, inode->i_ino);
 
+	if (!len)
+		return 0;
+
 	BUG_ON(end < lblk);
 
 	newes.es_lblk = lblk;
@@ -649,6 +652,9 @@ int ext4_es_remove_extent(struct inode *inode, ext4_lblk_t lblk,
 	es_debug("remove [%u/%u) from extent status tree of inode %lu\n",
 		 lblk, len, inode->i_ino);
 
+	if (!len)
+		return err;
+
 	end = lblk + len - 1;
 	BUG_ON(end < lblk);