x86, xen: degrade BUG to WARN when multicall fails

If one of the components of a multicall fails, WARN rather than BUG,
to help with debugging.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/multicalls.c

@@ -179,7 +179,7 @@ void xen_mc_flush(void)
 	}
 	b->cbidx = 0;
 
-	BUG_ON(ret);
+	WARN_ON(ret);
 }
 
 struct multicall_space __xen_mc_entry(size_t args)