jfs: add sanity check for agwidth in dbMount

The width in dmapctl of the AG is zero, it trigger a divide error when
calculating the control page level in dbAllocAG.

To avoid this issue, add a check for agwidth in dbAllocAG.

Reported-and-tested-by:  <syzbot+7c808908291a569281a9@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=7c808908291a569281a9


Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>