x86/mm: use "break" in arch_memory_op()

The final return statement is unreachable and hence disliked by Misra
C:2012 (rule 2.1). Convert those case-specific (main) return statements
which already use "rc", or in one case when it can be used without
further adding of code, to break.

No functional change intended.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/x86/mm.c

@@ -4724,7 +4724,7 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         spin_unlock(&d->arch.e820_lock);
 
         rcu_unlock_domain(d);
-        return rc;
+        break;
     }
 
     case XENMEM_memory_map:
@@ -4818,7 +4818,7 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         if ( __copy_to_guest(arg, &ctxt.map, 1) )
             return -EFAULT;
 
-        return 0;
+        break;
     }
 
     case XENMEM_machphys_mapping:
@@ -4880,7 +4880,7 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         }
 
         rcu_unlock_domain(d);
-        return rc;
+        break;
     }
 #endif
 
@@ -4888,7 +4888,7 @@ long arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         return subarch_memory_op(cmd, arg);
     }
 
-    return 0;
+    return rc;
 }
 
 int cf_check mmio_ro_emulated_write(