xfs: annotate user variables passed as void

Some argument callbacks can contain user buffers, and sparse warns
about passing them as void pointers. Cast appropriately to remove
the sparse warnings.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>