xfree(): simplify

Contrary to a popular opinion, there is no need to check
an argument for being non-NULL before calling free().

>From free(3) man page:

> > If ptr is NULL, no operation is performed.

Let's change xfree macro to be a synonym for free().

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>