Coverity Model Update

Coverity Model Update

When reviewing Clang Static Analyzer reports against a branch that had
experimental header changes based on the Coverity model file to inform
it that KM_SLEEP allocations cannot return NULL, I found a report saying
that a KM_PUSHPAGE allocation returned NULL. The actual implementation
does not return NULL unless KM_NOSLEEP has been passed, so we backport
the correction from the experimental header changes to the Coverity
model.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14210