[S390] remove ptrace warning on 31 bit.

A kernel compile on 31 bit gives the following warnings in ptrace.c:

arch/s390/kernel/ptrace.c: In function 'peek_user':
arch/s390/kernel/ptrace.c:207: warning: unused variable 'dummy'
arch/s390/kernel/ptrace.c: In function 'poke_user':
arch/s390/kernel/ptrace.c:315: warning: unused variable 'dummy'

Getting rid of the dummy variables removes the warnings.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/ptrace.c

@@ -204,7 +204,6 @@ static unsigned long __peek_user(struct task_struct *child, addr_t addr)
 static int
 peek_user(struct task_struct *child, addr_t addr, addr_t data)
 {
-	struct user *dummy = NULL;
 	addr_t tmp, mask;
 
 	/*
@@ -213,8 +212,8 @@ peek_user(struct task_struct *child, addr_t addr, addr_t data)
 	 */
 	mask = __ADDR_MASK;
 #ifdef CONFIG_64BIT
-	if (addr >= (addr_t) &dummy->regs.acrs &&
-	    addr < (addr_t) &dummy->regs.orig_gpr2)
+	if (addr >= (addr_t) &((struct user *) NULL)->regs.acrs &&
+	    addr < (addr_t) &((struct user *) NULL)->regs.orig_gpr2)
 		mask = 3;
 #endif
 	if ((addr & mask) || addr > sizeof(struct user) - __ADDR_MASK)
@@ -312,7 +311,6 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 static int
 poke_user(struct task_struct *child, addr_t addr, addr_t data)
 {
-	struct user *dummy = NULL;
 	addr_t mask;
 
 	/*
@@ -321,8 +319,8 @@ poke_user(struct task_struct *child, addr_t addr, addr_t data)
 	 */
 	mask = __ADDR_MASK;
 #ifdef CONFIG_64BIT
-	if (addr >= (addr_t) &dummy->regs.acrs &&
-	    addr < (addr_t) &dummy->regs.orig_gpr2)
+	if (addr >= (addr_t) &((struct user *) NULL)->regs.acrs &&
+	    addr < (addr_t) &((struct user *) NULL)->regs.orig_gpr2)
 		mask = 3;
 #endif
 	if ((addr & mask) || addr > sizeof(struct user) - __ADDR_MASK)