Don't allow a group to be converted to POSIX and external
This condition was checked in group-add but not in group-mod.
This evaluation is done later in the pre_callback so that all
the other machinations about posix are already done to make
it easier to tell whether this condition is true or not.
Fixes: https://pagure.io/freeipa/issue/8990
Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>