fix issue with checksum calculation. FileChannel has to be set to 0th
position before checksum is calculated

Require-book: no
Require-notes: yes
Bug: http://rt.dcache.org/Ticket/Display.html?id=7579
Acked-by: Tigran Mkrtchyan
Target: trunk
Request: 2.2
Request: 2.4
Request: 2.5