Bluetooth: Fix potential double free caused by hci_conn_unlink

Bluetooth: Fix potential double free caused by hci_conn_unlink

The hci_conn_unlink function is being called by hci_conn_del, which
means it should not call hci_conn_del with the input parameter conn
again. If it does, conn may have already been released when
hci_conn_unlink returns, leading to potential UAF and double-free
issues.

This patch resolves the problem by modifying hci_conn_unlink to release
only conn's child links when necessary, but never release conn itself.

Reported-by: syzbot+690b90b14f14f43f4688@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-bluetooth/000000000000484a8205faafe216@google.com/
Fixes: 06149746e720 ("Bluetooth: hci_conn: Add support for linking multiple hcon")
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reported-by: syzbot+690b90b14f14f43f4688@syzkaller.appspotmail.com
Reported-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Reported-by: syzbot+8bb72f86fc823817bc5d@syzkaller.appspotmail.com