BUG/MINOR: mworker: always stop the receiving listener

BUG/MINOR: mworker: always stop the receiving listener

Upon _send_status, always stop the listener from which the request
was received, rather than looking it up from the proc_list entry via
fdtab[proc->ipc_fd[0]].owner.

A BUG_ON is added to verify that the listener which received the
request is the one expected for the reported PID.

This means it is no longer possible to send "_send_status READY XXX"
manually through the master CLI for testing, as that would trigger
the BUG_ON.

Must be backported as far as 3.1.