xen/page_alloc: address violations of MISRA C Rule 20.7

MISRA C Rule 20.7 states: "Expressions resulting from the expansion
of macro parameters shall be enclosed in parentheses". Therefore, some
macro definitions should either gain additional parentheses or have
their expressions simplified to ensure that all current and future users
will be safe with respect to expansions that can possibly alter the
semantics of the passed-in macro parameter.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/common/page_alloc.c

@@ -150,7 +150,7 @@
 #include <asm/paging.h>
 #else
 #define p2m_pod_offline_or_broken_hit(pg) 0
-#define p2m_pod_offline_or_broken_replace(pg) BUG_ON(pg != NULL)
+#define p2m_pod_offline_or_broken_replace(pg) BUG_ON(pg)
 #endif
 
 #ifndef PGC_static