ia32/vdso: pretify helper for mmaping 32bit vDSO

Forward CRIU's log fd into helper, so it can actually print errors.
This will help to debug troubles with helper, like #273.
Log fd can be safely written to, as it's opened in CRIU with
O_APPEND or it's line-buffered (stdout) or not buffered (stderr)
stream. Anyway, it's double safe, as there is simple synchronization
between helper and CRIU, so only one of them will log at a time.
Also expanded the comment about a helper.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>