jfs: upper bound check of tree index in dbAllocAG

[ Upstream commit c214006856ff52a8ff17ed8da52d50601d54f9ce ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>