Remove macro definition and the remaining uses.
RETURN() macro is just a wrapper for goto, most of the time it makes the code more complicated than using goto directly.
According to JIRA user simonelombardo, there was crash in exiting a thread.
- Add UserAssignmentLock, and UserAssignmentUnlock helper functions.
- Lock and unlock pti->spDefaultImc by using those helper functions.
CORE-18044