[S390] add missing .set function for NT_S390_LAST_BREAK regset

The last breaking event address is a read-only value, the regset misses the
.set function. If a PTRACE_SETREGSET is done for NT_S390_LAST_BREAK we
get an oops due to a branch to zero:

Kernel BUG at 0000000000000002 verbose debug info unavailable
illegal operation: 0001 #1 SMP
...
Call Trace:
(<0000000000158294> ptrace_regset+0x184/0x188)
<00000000001595b6> ptrace_request+0x37a/0x4fc
<0000000000109a78> arch_ptrace+0x108/0x1fc
<00000000001590d6> SyS_ptrace+0xaa/0x12c
<00000000005c7a42> sysc_noemu+0x16/0x1c
<000003fffd5ec10c> 0x3fffd5ec10c
Last Breaking-Event-Address:
<0000000000158242> ptrace_regset+0x132/0x188

Add a nop .set function to prevent the branch to zero.

Signed-off-by: Martin Schwidefsky <[e-mail]>
Cc: [e-mail]

arch/s390/kernel/ptrace.c

···
 	return 0;
 }
 
+static int s390_last_break_set(struct task_struct *target,
+			       const struct user_regset *regset,
+			       unsigned int pos, unsigned int count,
+			       const void *kbuf, const void __user *ubuf)
+{
+	return 0;
+}
+
 #endif
 
 static int s390_system_call_get(struct task_struct *target,
···
 		.size = sizeof(long),
 		.align = sizeof(long),
 		.get = s390_last_break_get,
+		.set = s390_last_break_set,
 	},
 #endif
 	[REGSET_SYSTEM_CALL] = {
···
 	return 0;
 }
 
+static int s390_compat_last_break_set(struct task_struct *target,
+				      const struct user_regset *regset,
+				      unsigned int pos, unsigned int count,
+				      const void *kbuf, const void __user *ubuf)
+{
+	return 0;
+}
+
 static const struct user_regset s390_compat_regsets[] = {
 	[REGSET_GENERAL] = {
 		.core_note_type = NT_PRSTATUS,
···
 		.size = sizeof(long),
 		.align = sizeof(long),
 		.get = s390_compat_last_break_get,
+		.set = s390_compat_last_break_set,
 	},
 	[REGSET_SYSTEM_CALL] = {
 		.core_note_type = NT_S390_SYSTEM_CALL,