jfs: upper bound check of tree index in dbAllocAG

When computing the tree index in dbAllocAG, we never check if we are
out of bounds realative to the size of the stree.
This could happen in a scenario where the filesystem metadata are
corrupted.

Reported-by:  <syzbot+cffd18309153948f3c3e@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=cffd18309153948f3c3e


Tested-by:  <syzbot+cffd18309153948f3c3e@syzkaller.appspotmail.com>
Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>