kerndat: do not report error on loginuid feature test

Fix for commit 0ce8e429 ("kerndat: do not report errors on feature
test").
That commit hid error messages for feature testing when you cannot
write to /proc/*/loginuid files because of missing kernel patch that
allows unsetting loginuid value on older kernels, but it didn't hide
error messages in case of disabled CONFIG_AUDITSYSCALL - then you
don't have loginuid files.
Also fixed comment for kerndat feature test: procfs file might fail
to open if it's missing and that's fine - !CONFIG_AUDITSYSCALL case,
but it can't fail due permission fault on _read_ (then something is
wrong, lets report a problem).

Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>