sadf: Update conversion function

sadf: Update conversion function

Update sadf function used to convert an old datafile (from version 9.1.6
and later) to the up-to-date format (11.7.2). This function corresponds
to sadf option -c ("sadf -c old_datafile > new_datafile").
It should work on both little endian and big endian data files.
The original endianness of the file is preserved.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>