Honor SBufReservationRequirements::minSize regardless of idealSize.

Honor SBufReservationRequirements::minSize regardless of idealSize.

  In a fully specified SBufReservationRequirements, idealSize would
  naturally match or exceed minSize. However, the idealSize default value
  (zero) may not. We should honor minSize regardless of idealSize, just as
  the API documentation promises to do.

  No runtime changes expected right now because the only existing user of
  SBufReservationRequirements sets .idealSize to CLIENT_REQ_BUF_SZ (4096)
  and .minSize to 1024.