x86/mem-sharing: short-circuit p2m_is_shared() when MEM_SHARING=n

Some of the uses of dom_cow aren't easily DCE-able (without extra
#ifdef-ary), and hence it being constantly NULL when MEM_SHARING=n
misguides Coverity into thinking that there may be a NULL deref in

        if ( p2m_is_shared(t) )
            d = dom_cow;

        if ( get_page(page, d) )
            return page;

(in get_page_from_mfn_and_type()). Help the situation by making
p2m_is_shared() be compile-time false when MEM_SHARING=n, thus also
permitting the compiler to DCE some other code.

Note that p2m_is_sharable() isn't used outside of mem_sharing.c, and
hence P2M_SHARABLE_TYPES can simply be left undefined when
MEM_SHARING=n.

Coverity ID: 1645573
Fixes: 79d91e178a ("dom_cow is needed for mem-sharing only")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tamas K Lengyel <tamas@tklengyel.com>

xen/arch/x86/include/asm/p2m.h

@@ -136,11 +136,16 @@ typedef unsigned int p2m_query_t;
 #endif
 
 /* Shared types */
+#ifdef CONFIG_MEM_SHARING
 /* XXX: Sharable types could include p2m_ram_ro too, but we would need to
  * reinit the type correctly after fault */
 #define P2M_SHARABLE_TYPES (p2m_to_mask(p2m_ram_rw) \
                             | p2m_to_mask(p2m_ram_logdirty) )
 #define P2M_SHARED_TYPES   (p2m_to_mask(p2m_ram_shared))
+#else
+/* P2M_SHARABLE_TYPES deliberately not provided. */
+#define P2M_SHARED_TYPES 0
+#endif
 
 /* Types established/cleaned up via special accessors. */
 #define P2M_SPECIAL_TYPES (P2M_GRANT_TYPES | \