Pass fflag down to g_ctl_req() and:
- Refuse all requests unless fflag has the FREAD bit set
- Refuse all but getxml unless fflag also has the FWRITE bit set
This should allow us to restore read access to /dev/geom.ctl for group
operator; they will then be able to run gpart list or gpart backup
but not commands that actually modify the table.
MFC after: 1 week
Event: EuroBSDCon 2026 DevSummit