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

CID 743396 (#1 of 1): Unchecked return value from library (CHECKED_RETURN)
  check_return: Calling function "posix_fadvise(fd, offset, bl->length(), 4)"
   without checking return value. This library function may fail and return
   an error code.
  unchecked_value: No check of the return value of
   "posix_fadvise(fd, offset, bl->length(), 4)".

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