jfs: add check read-only before truncation in jfs_truncate_nolock()

Added a check for "read-only" mode in the `jfs_truncate_nolock`
function to avoid errors related to writing to a read-only
filesystem.

Call stack:

block_write_begin() {
  jfs_write_failed() {
    jfs_truncate() {
      jfs_truncate_nolock() {
        txEnd() {
          ...
          log = JFS_SBI(tblk->sb)->log;
          // (log == NULL)

If the `isReadOnly(ip)` condition is triggered in
`jfs_truncate_nolock`, the function execution will stop, and no
further data modification will occur. Instead, the `xtTruncate`
function will be called with the "COMMIT_WMAP" flag, preventing
modifications in "read-only" mode.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by:  <syzbot+4e89b5368baba8324e07@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=4e89b5368baba8324e07


Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>