namespace: use ferror() to detect errors of fscanf()

Currently we check errno, but musl-libc doesn't zero it,
if fscanf() has returned EOF without errors.

travis-ci: success for A few fixes for Alpine Linux
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>