x86: fix build with gcc 7

-Wint-in-bool-context, enabled by default in gcc 7, doesn't like
multiplication in conditional operators. "Hide" them, at the risk of
the next compiler version becoming smarter and recognizing even those.
(The hope is that added smartness then would also better deal with
legitimate cases like the ones here.)

The change could have been done in access_ok(), but I think we better
keep it at the places the compiler is actually unhappy about.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: f32400e90c
master date: 2017-05-19 10:11:36 +0200

xen/arch/x86/traps.c

@@ -3169,7 +3169,7 @@ static void emulate_gate_op(struct cpu_user_regs *regs)
                 return;
             }
             stkp = (unsigned int *)(unsigned long)((unsigned int)base + esp);
-            if ( !compat_access_ok(stkp - 4 - nparm, (4 + nparm) * 4) )
+            if ( !compat_access_ok(stkp - 4 - nparm, 16 + nparm * 4) )
             {
                 do_guest_trap(TRAP_gp_fault, regs, 1);
                 return;
@@ -3188,7 +3188,7 @@ static void emulate_gate_op(struct cpu_user_regs *regs)
                      !check_stack_limit(ar, limit, esp + nparm * 4, nparm * 4) )
                     return do_guest_trap(TRAP_gp_fault, regs, 1);
                 ustkp = (unsigned int *)(unsigned long)((unsigned int)base + regs->_esp + nparm * 4);
-                if ( !compat_access_ok(ustkp - nparm, nparm * 4) )
+                if ( !compat_access_ok(ustkp - nparm, 0 + nparm * 4) )
                 {
                     do_guest_trap(TRAP_gp_fault, regs, 1);
                     return;

xen/include/asm-x86/x86_64/uaccess.h

@@ -55,7 +55,7 @@ extern void *xlat_malloc(unsigned long *xlat_page_current, size_t size);
 
 #define compat_array_access_ok(addr,count,size) \
     (likely((count) < (~0U / (size))) && \
-     compat_access_ok(addr, (count) * (size)))
+     compat_access_ok(addr, 0 + (count) * (size)))
 
 #define __put_user_size(x,ptr,size,retval,errret)			\
 do {									\