s390/ftrace: fix mcount adjustment

Tony Jones reported that the ftrace self tests on s390 do not work:

<6>Testing dynamic ftrace ops #1: (0 0 0 0 0) FAILED!
<6>Testing tracer irqsoff:
<3>failed to start irqsoff tracer
<4>.. no entries found ..FAILED!
<6>Testing tracer wakeup:
<3>failed to start wakeup tracer
<4>.. no entries found ..FAILED!
<6>Testing tracer function_graph:
<4>Failed to init function_graph tracer, init returned -19
<4>FAILED!

This happens because we forgot to adjust the instruction pointer that gets
passed to the ftrace trace function by MCOUNT_INSN_SIZE.

In addition change MCOUNT_INSN_SIZE to the correct value on 31 bit.
It only worked so far because the to be patched instruction was identical.

Reported-by: Tony Jones <tonyj@suse.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/mcount.S

@@ -7,6 +7,7 @@
 
 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
+#include <asm/ftrace.h>
 
 	.section .kprobes.text, "ax"
 
@@ -33,6 +34,7 @@ ENTRY(ftrace_caller)
 	la	%r2,0(%r14)
 	st	%r0,__SF_BACKCHAIN(%r15)
 	la	%r3,0(%r3)
+	ahi	%r2,-MCOUNT_INSN_SIZE
 	l	%r14,0b-0b(%r1)
 	l	%r14,0(%r14)
 	basr	%r14,%r14