FileJournal: reduce time wasted by bufferptr::zero

FileJournal: reduce time wasted by bufferptr::zero

bufferptr::zero(), apart from actual memset() call, also invalidates its
internal CRC cache, which is empty at the time, so no point to lock its
mutex, clear CRC cache, unlock mutex and memset() entire bufferptr. Also,
not entire bufferptr needs to be zeroed, so clear out just unused parts
of it.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>