Bluetooth: hci_conn: fix potential UAF in create_big_sync

Add hci_conn_valid() check in create_big_sync() to detect stale
connections before proceeding with BIG creation. Handle the
resulting -ECANCELED in create_big_complete() and re-validate the
connection under hci_dev_lock() before dereferencing, matching the
pattern used by create_le_conn_complete() and create_pa_complete().

Keep the hci_conn object alive across the async boundary by taking
a reference via hci_conn_get() when queueing create_big_sync(), and
dropping it in the completion callback. The refcount and the lock
are complementary: the refcount keeps the object allocated, while
hci_dev_lock() serializes hci_conn_hash_del()'s list_del_rcu() on
hdev->conn_hash, as required by hci_conn_del().

hci_conn_put() is called outside hci_dev_unlock() so the final put
(which resolves to kfree() via bt_link_release) does not run under
hdev->lock, though the release path would be safe either way.

Without this, create_big_complete() would unconditionally
dereference the conn pointer on error, causing a use-after-free
via hci_connect_cfm() and hci_conn_del().

Fixes: eca0ae4aea66 ("Bluetooth: Add initial implementation of BIS connections")
Cc: stable@vger.kernel.org
Co-developed-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>