sysctl: restore sysctls for correct namespaces

When we don't use userns, __userns_sysctl_op is called
in context of the current process. A mount namespaces is restored
the last one, so when we restore namespaces, we see /proc from the
host pid namespace. In this case we can't use virtual pid to access
/proc/pid.

Let's open /proc/self/ns and use this descriptor to switch namespaces.

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Fixes: f79f4546 ("sysctl: move sysctl calls to usernsd")
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>