Changing integer sizes of n and iwlen to prevent overflow

[ashermancinelli]

Changed relevant calls to metis functions to match new types. @flipflapflop Thank you for your responsiveness on this issue, your review is appreciated.

Steps:

1. I built metis 5.1.0 with this patch
2. Configured with: ../configure --with-metis="-L$INSTALL_LOCAL/lib -lmetis" --with-blas="-L$INSTALL_LOCAL/lib -lopenblas" --prefix=$INSTALL_LOCAL pointing to my local installations of openblas and metis
3. Tested with matrix bcsstk39.rb
4. Ran with command spral_ssids matrix.rb --ngpus 1 --min-gpu-work 0 in a slurm allocation with one node and one process