[HOTFIX] Bug fix in triangle counting

[ChuckHastings]

Optimizations in the triangle counting logic did computations using available memory to determine the number of cuda blocks to use.

Computation was using a 32-bit integer and on an 80 GB memory system would overflow and generate a negative number - causing a crash.

Adjusted the computation to be done as size_t and cast to an int after determining the number of blocks.

[codecov-commenter]

Codecov Report

Merging #1654 (2d79697) into branch-21.06 (57ee866) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.06    #1654      +/-   ##
================================================
- Coverage         59.79%   59.77%   -0.03%     
================================================
  Files                80       80              
  Lines              3542     3540       -2     
================================================
- Hits               2118     2116       -2     
  Misses             1424     1424              

Impacted Files	Coverage Δ
python/cugraph/_version.py	44.40% <0.00%> (-0.40%)	⬇️

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