firewire: core: fix race condition in split transaction handling

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

firewire: core: fix race condition in split transaction handling

For a split transaction, either fw_core_handle_response() or
split_transaction_timeout_callback() may manipulate the same transaction
entry in the pending transaction list. The return value of timer_delete()
is used to determine which function handles the entry. If the transaction
is a split transaction and timer_delete() returns zero, the timeout
callback may already be running. In this case, the response handler must
leave the entry in the list for the timeout callback to remove.

A previous code refactoring broke this behavior, introducing a race
condition in which both functions could process the same transaction.
This could lead to a use-after-free on the transaction initiator side.

Fix the regression by ensuring that the response handler does not process
the transaction when timer_delete() returns zero. The timeout callback is
responsible for removing the entry from the pending transaction list.