jfs: fix array-index-out-of-bounds read in add_missing_indices

stbl is s8 but it must contain offsets into slot which can go from 0 to
127.

Added a bound check for that error and return -EIO if the check fails.
Also make jfs_readdir return with error if add_missing_indices returns
with an error.

Reported-by:  <syzbot+b974bd41515f770c608b@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com./bug?extid=b974bd41515f770c608b


Signed-off-by: Aditya Dutt <duttaditya18@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>