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>

include/xmalloc.h

@@ -20,7 +20,7 @@
 #define xzalloc(size)		__xalloc(calloc, size, 1, size)
 #define xrealloc(p, size)	__xalloc(realloc, size, p, size)
 
-#define xfree(p)		do { if (p) free(p); } while (0)
+#define xfree(p)		free(p)
 
 #define xrealloc_safe(pptr, size)					\
 	({								\