FS: JFS: Fix null-ptr-deref Read in txBegin

In the Linux kernel, the following vulnerability has been resolved: FS: JFS: Fix null-ptr-deref Read in txBegin Syzkaller reported an issue where txBegin may be called on a superblock in a read-only mounted filesystem which leads to NULL pointer deref. This could be solved by checking if the filesystem is read-only before calling txBegin, and returning with appropiate error code.