Btrfs: disallow 'btrfs {balance,replace} cancel' on ro mounts

For both balance and replace, cancelling involves changing the on-disk
state and committing a transaction, which is not a good thing to do on
read-only filesystems.

Cc: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/dev-replace.c

@@ -650,6 +650,9 @@ static u64 __btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 	u64 result;
 	int ret;
 
+	if (fs_info->sb->s_flags & MS_RDONLY)
+		return -EROFS;
+
 	mutex_lock(&dev_replace->lock_finishing_cancel_unmount);
 	btrfs_dev_replace_lock(dev_replace);
 	switch (dev_replace->replace_state) {

fs/btrfs/volumes.c

@@ -3424,6 +3424,9 @@ int btrfs_pause_balance(struct btrfs_fs_info *fs_info)
 
 int btrfs_cancel_balance(struct btrfs_fs_info *fs_info)
 {
+	if (fs_info->sb->s_flags & MS_RDONLY)
+		return -EROFS;
+
 	mutex_lock(&fs_info->balance_mutex);
 	if (!fs_info->balance_ctl) {
 		mutex_unlock(&fs_info->balance_mutex);