jfs: upper bound check of tree index in dbAllocAG

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>