sysctl: Don't forget to close fds

In case if openat/open call returned fd = 0,
don't forget to close it on routine exit,
ie just use close_safe macro.

Not a big deal since the descriptors get closed
once program finish execution but still.

#CID 996183 Resource leak

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>