In apm_write(), both fd and imgsz are referenced, so don't mark the
arguments as unused.
Due to reasons unknown at this time, the system can be forced to write
a journal block even when there are no journal entries to be written.
Until the root cause is found, handle this case by ensuring that a
valid journal segment is always written.

Second, the data buffer used for writing journal entries was never
being scrubbed of old data.  Fix this.

Submitted by:	Takehara Mikihito
Obtained from:	Netflix, Inc.
MFC after:	3 days