Standardize RHEL version check in packages

This is a follow up to 3c356622994 which standardizes how the RHEL
version check is done. This simpler "0%{?rhel}" check is used
elsewhere in the packages so we do the same here.

Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13501