in-kernel PM: closed subflows before `RM_ADDR` will not decrement `add_addr_accepted`

[matttbe]

(Linked to #496 - maybe can be marked as fixed once this modification is in place if we are OK to take this direction)

If subflows are removed from the list of subflows (closed from the other peer, error, never established, etc.) before receiving a RM_ADDR, the RM_ADDR will not decrement the add_addr_accepted counter if there is no other subflows using the remote ID from the RM_ADDR.

In other words, when subflows are getting closed, the in-kernel PM might need to do some extra actions, like decrementing the add_addr_accepted counter if there is no other subflows using the remote ID of the ones getting removed.

[geliangtang]

Hi Matt, I assigned this issue to myself since my colleague Gang Yan is looking at it.