Follow-up r297156: Close the log in filemon_dtr rather than in the last reference.

If the tracer has decided to the close the log then it should be fully
written, not getting more entries, when close(2) returns. This was
a regression in r297156 in that it allowed a traced process to continue
a traced syscall and add more entries to the log while the tracer had
already closed its fd or exited. This was only really part of the
daemonized process case which is abnormal.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division