src/rbd.cc: use 64-bits to shift 'order'

CID 1019568 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
  overflow_before_widen: Potentially overflowing expression "1 << *order" with
  type "int" (32 bits, signed) is evaluated using 32-bit arithmetic before being
  used in a context which expects an expression of type "uint64_t" (64 bits,
  unsigned). To avoid overflow, cast the left operand to "uint64_t" before
  performing the left shift.

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