Fix kmodtool for packaging mainline Linux

kmodtool currently incorrectly identifies official
RHEL kernels, as opposed to custom kernels. This
can cause the openZFS kmod RPM build to break.

The issue can be reproduced by building a set of
mainline Linux RPMs, installing them, and then
attempting to build the openZFS kmod package
against them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Timothy Day <timday@amazon.com>
Closes #14617