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

Currently there is not check against the agno of the iag while
allocating new inodes to avoid fragmentation problem. Added the check
which is required.

Reported-by:  <syzbot+79d792676d8ac050949f@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=79d792676d8ac050949f
Signed-off-by: Manas Ghandat <ghandatmanas@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>