library: expand VAL macros to include the context parm

library: expand VAL macros to include the context parm

This patch will set the stage for validating the types
referenced in the result union. For now, the parameter
representing that 'info' structure will remain unused.

[ and while we're at it, let us correct a faulty GET ]
[ macro in the diskstats header. that puppy missed a ]
[ parm which ain't so good if that guy is ever used! ]

Signed-off-by: Jim Warner <james.warner@comcast.net>