CVE-2026-98459: Bluetooth: ISO: balance the parent hold in hci_bind_bis()

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: ISO: balance the parent hold in hcibindbis()

hciconnlink() takes a lifetime reference to its parent with hciconnget(), but only takes an operational hold on the child. hciconnunlink() later balances both a hold and a reference on the parent.

The SCO and CIS paths pass a parent acquired from a connect helper, so it already has a hold. For an additional BIS, hcibindbis() obtains the parent from hciconnhashlookupbig(), which returns a bare pointer. Unlinking the child then drops the parent's existing hold and can schedule it for disconnection while its socket is still using it.

Take a hold on the parent before linking it and drop that hold if linking fails. A successful link transfers the hold to hciconnunlink().