Add coverity model to repository

Other projects such as the python project include their coverity models
in their repositories. This provides transparency, which is beneficial
in open source projects. Therefore, it is a good idea to include the
coverity model in our repository too.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13884