ubifs: read-only if LEB may always be taken in ubifs_garbage_collect

If ubifs_garbage_collect_leb() returns -EAGAIN and ubifs_return_leb
returns error, a LEB will always has a "taken" flag. In this case,
set the ubifs to read-only to prevent a worse situation.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>