bench/dumb_backend.cc: check return value of lseek()

CID 743395 (#1 of 1): Unchecked return value from library (CHECKED_RETURN)
  check_return: Calling function "lseek(fd, offset, 0)" without checking
   return value. This library function may fail and return an error code.
  unchecked_value: No check of the return value of "lseek(fd, offset, 0)".

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