x86/pv: fix clang build without CONFIG_PV32

Clang reports the following build error without CONFIG_PV32:

hypercall.c:253:10: error: variable 'op' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    if ( !is_pv_32bit_vcpu(curr) )
         ^~~~~~~~~~~~~~~~~~~~~~~
hypercall.c:282:21: note: uninitialized use occurs here
    return unlikely(op == __HYPERVISOR_iret)
                    ^~
/root/src/xen/xen/include/xen/compiler.h:21:43: note: expanded from macro 'unlikely'
#define unlikely(x)   __builtin_expect(!!(x),0)
                                          ^
hypercall.c:253:5: note: remove the 'if' if its condition is always true
    if ( !is_pv_32bit_vcpu(curr) )
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hypercall.c:251:21: note: initialize the variable 'op' to silence this warning
    unsigned long op;
                    ^
                     = 0

Rearrange the code in arch_do_multicall_call so that the if guards the
32bit branch and when CONFIG_PV32 is not set there's no conditional at
all.

Fixes: 527922008b ('x86: slim down hypercall handling when !PV32')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/pv/hypercall.c

@@ -250,21 +250,8 @@ enum mc_disposition arch_do_multicall_call(struct mc_state *state)
     struct vcpu *curr = current;
     unsigned long op;
 
-    if ( !is_pv_32bit_vcpu(curr) )
-    {
-        struct multicall_entry *call = &state->call;
-
-        op = call->op;
-        if ( (op < ARRAY_SIZE(pv_hypercall_table)) &&
-             pv_hypercall_table[op].native )
-            call->result = pv_hypercall_table[op].native(
-                call->args[0], call->args[1], call->args[2],
-                call->args[3], call->args[4], call->args[5]);
-        else
-            call->result = -ENOSYS;
-    }
 #ifdef CONFIG_PV32
-    else
+    if ( is_pv_32bit_vcpu(curr) )
     {
         struct compat_multicall_entry *call = &state->compat_call;
 
@@ -277,7 +264,20 @@ enum mc_disposition arch_do_multicall_call(struct mc_state *state)
         else
             call->result = -ENOSYS;
     }
+    else
 #endif
+    {
+        struct multicall_entry *call = &state->call;
+
+        op = call->op;
+        if ( (op < ARRAY_SIZE(pv_hypercall_table)) &&
+             pv_hypercall_table[op].native )
+            call->result = pv_hypercall_table[op].native(
+                call->args[0], call->args[1], call->args[2],
+                call->args[3], call->args[4], call->args[5]);
+        else
+            call->result = -ENOSYS;
+    }
 
     return unlikely(op == __HYPERVISOR_iret)
            ? mc_exit