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>

Author: ea1davis

fs/jfs/jfs_dmap.c

@@ -204,6 +204,10 @@ int dbMount(struct inode *ipbmap)
 	bmp->db_aglevel = le32_to_cpu(dbmp_le->dn_aglevel);
 	bmp->db_agheight = le32_to_cpu(dbmp_le->dn_agheight);
 	bmp->db_agwidth = le32_to_cpu(dbmp_le->dn_agwidth);
+	if (!bmp->db_agwidth) {
+		err = -EINVAL;
+		goto err_release_metapage;
+	}
 	bmp->db_agstart = le32_to_cpu(dbmp_le->dn_agstart);
 	bmp->db_agl2size = le32_to_cpu(dbmp_le->dn_agl2size);
 	if (bmp->db_agl2size > L2MAXL2SIZE - L2MAXAG ||