Update coverity model

Update coverity model

`uu_panic()` needs to be modelled and the definition of `vpanic()` from
the original coverity model was missing
`__coverity_format_string_sink__()`.

We also model `libspl_assertf()` as part of an attempt to eliminate
false positives.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13901