sysctl: Use open_proc()

Many of sysctl_op-s we have read (or write) single entry.
In current implementaiton this results in two opens and
two closes for each -- open /proc/sys, then open the rest.

It's better to use open_proc() as the latter already have
fd for /proc cached.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>