netconsole: implement configfs for release_enabled

Implement the configfs helpers to show and set release_enabled configfs
directories under userdata.

When enabled, set the feature bit in netconsole_target->sysdata_fields.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250314-netcons_release-v1-2-07979c4b86af@debian.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>