libblkid,libmount: Do not use void* in calculations [cppcheck]

libblkid,libmount: Do not use void* in calculations [cppcheck]

[libblkid/src/superblocks/zfs.c:179]: (portability) 'label' is of type 'const void *'. When using void pointers in calculations, the behaviour is undefined.
[libblkid/src/superblocks/zfs.c:237]: (portability) 'label' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.
[libblkid/src/topology/topology.c:221]: (portability) 'chn.data' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.
[libmount/src/fs.c:153]: (portability) 'old' is of type 'const void *'. When using void pointers in calculations, the behaviour is undefined.
[libmount/src/fs.c:154]: (portability) 'new' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.