jfs: fix array-index-out-of-bounds in jfs_readdir

[ Upstream commit 839f102efb168f02dfdd46717b7c6dddb26b015e ]

The stbl might contain some invalid values. Added a check to
return error code in that case.

Reported-by:  <syzbot+0315f8fe99120601ba88@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=0315f8fe99120601ba88


Signed-off-by: Ghanshyam Agrawal <ghanshyam1898@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>