libfdisk: cleanup sun label checksum usuage

libfdisk: cleanup sun label checksum usuage

We are using now the formerly unused function sun_pt_checksum(). This
cleanup was motivated by clang compiler warning, see below. Also nice
that we are now always using uint16_t instead of short.

Warning was:

../libfdisk/src/sun.c:177:35: warning: taking address of packed member 'csum' of class or structure 'sun_disklabel' may result in an unaligned pointer value
      [-Waddress-of-packed-member]
                while(ush < (unsigned short *)(&sunlabel->csum))

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>