osdc/Journaler.cc: fix integer overflow

osdc/Journaler.cc: fix integer overflow

Fix for:

CID 1297861 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
 overflow_before_widen: Potentially overflowing expression
  this->layout.fl_stripe_count.operator __u32() *
  this->layout.fl_object_size.operator __u32() with type unsigned int
  (32 bits, unsigned) is evaluated using 32-bit arithmetic, and then used
  in a context that expects an expression of type uint64_t (64 bits, unsigned).

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>