s390/perf: fix gcc 8 array-bounds warning

arch/s390/kernel/perf_regs.c:36:19: warning: array subscript 16 is above
array bounds of 'long unsigned int[16]' [-Warray-bounds]
  return regs->gprs[idx];

gcc tries to be smart here and since there is a condition:
if (idx >= PERF_REG_S390_R0 && idx <= PERF_REG_S390_R15)
	return regs->gprs[idx];
which covers all possible array subscripts, it gives the warning
for the last function return statement:
	return regs->gprs[idx];
which in presence of that condition does not really make sense and
should be replaced with "return 0;"

Also move WARN_ON_ONCE((u32)idx >= PERF_REG_S390_MAX) to the end of the
function.
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/perf_regs.c

@@ -12,9 +12,6 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
 {
 	freg_t fp;
 
-	if (WARN_ON_ONCE((u32)idx >= PERF_REG_S390_MAX))
-		return 0;
-
 	if (idx >= PERF_REG_S390_R0 && idx <= PERF_REG_S390_R15)
 		return regs->gprs[idx];
 
@@ -33,7 +30,8 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
 	if (idx == PERF_REG_S390_PC)
 		return regs->psw.addr;
 
-	return regs->gprs[idx];
+	WARN_ON_ONCE((u32)idx >= PERF_REG_S390_MAX);
+	return 0;
 }
 
 #define REG_RESERVED (~((1UL << PERF_REG_S390_MAX) - 1))