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>