Clean up initial FileChannel writer reset's read

Clean up initial FileChannel writer reset's read

The code here bears marks of 'copy, paste and cudgel to compliance'
programming and is strikingly similar to what MappedJournalSegmentWriter
does -- except it needs to deal with reading as well.

This patch takes the first step towards sanity by eliminating a useless
if() and uses FileChannel.read(ByteBuffer, long) to reduce position
adjustments.

JIRA: CONTROLLER-2095
Change-Id: I528cb14608402a406e20d168a7e8520c9eb66edf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>