exit1: postpone clearing P_TRACED flag until the proctree lock is acquired

In case of the process being debugged. The P_TRACED is cleared very early,
which would make procdesc_close() not calling proc_clear_orphan().
That would result in the debugged process can not be able to collect
status of the process with process descriptor.

Reviewed by: markj, kib
Tested by: pho
MFC after: 1 month