ext4: prevent delalloc to nodelalloc on remount

Implemented the suggested solution mentioned in the bug
https://bugzilla.kernel.org/show_bug.cgi?id=218820

Preventing the disabling of delayed allocation mode on remount.
delalloc to nodelalloc not permitted anymore
nodelalloc to delalloc permitted, not affected

Signed-off-by: Nicolas Bretz <bretznic@gmail.com>
Link: https://patch.msgid.link/20241014034143.59779-1-bretznic@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: nicbretz

fs/ext4/super.c

@@ -6528,6 +6528,13 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 		goto restore_opts;
 	}
 
+	if ((old_opts.s_mount_opt & EXT4_MOUNT_DELALLOC) &&
+	    !test_opt(sb, DELALLOC)) {
+		ext4_msg(sb, KERN_ERR, "can't disable delalloc during remount");
+		err = -EINVAL;
+		goto restore_opts;
+	}
+
 	sb->s_flags = (sb->s_flags & ~SB_POSIXACL) |
 		(test_opt(sb, POSIX_ACL) ? SB_POSIXACL : 0);