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