ext4: update c/mtime on truncate up

Commit 3da40c7b0898 ("ext4: only call ext4_truncate when size <= isize")
introduced a bug that c/mtime is not updated on truncate up.

Fix the issue by setting c/mtime explicitly in the truncate up case.

Note that ftruncate(2) is not affected, so you won't see this bug using
truncate(1) and xfs_io(1).

Signed-off-by: Zirong Lang <zorro.lang@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -4725,6 +4725,14 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
 				error = ext4_orphan_add(handle, inode);
 				orphan = 1;
 			}
+			/*
+			 * Update c/mtime on truncate up, ext4_truncate() will
+			 * update c/mtime in shrink case below
+			 */
+			if (!shrink) {
+				inode->i_mtime = ext4_current_time(inode);
+				inode->i_ctime = inode->i_mtime;
+			}
 			down_write(&EXT4_I(inode)->i_data_sem);
 			EXT4_I(inode)->i_disksize = attr->ia_size;
 			rc = ext4_mark_inode_dirty(handle, inode);