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.