xfb: Fix max_varyings/max_xfb_varyings mix-up.

The transform feedback max-varyings test had a mix-up between
max_varyings (the number of varying vec4's supported by the
implementation) and max_xfb_varyings (the number of transform feedback
output vec4's supported by the implementation).

As a result, we would run the test as though both values were
max_xfb_varyings, but we would check the results as though both values
were max_varyings.  This would cause spurious failures on
implementations for which max_varyings is greater than
max_xfb_varyings.