Bluetooth: controller: split: Fix ctrl proc rx node starvation

Rx nodes are reserved during control procedures for
generation of HCI event on completion. Fix the allocation
and reservation in the form of a linked list per connection
context. Worst case, this list will hold one rx node for
overlapping non-instant control procedure (Data Length
Update) plus two rx node for control procedure with instant
(PHY update with Data Length Update support).

Fixes #19198.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>