Fix BUG_ON() reporting in real mode on powerpc

[Balbir Singh]

>> Changelog:
>>      Don't add PAGE_OFFSET blindly, check if REGION_ID is 0
>>
>> I ran into this issue while debugging an early boot problem.
>> The system hit a BUG_ON() but report bug failed to print the
>> line number and file name. The reason being that the system
>> was running in real mode and report_bug() searches for
>> addresses in the PAGE_OFFSET+ region
>>
>> Suggested-by: Paul Mackerras <paulus at samba.org>
>> Signed-off-by: Balbir Singh <bsingharora at gmail.com>

<snip>

> Can we add some comments around this. When i looked at this first, i was
> wondering how nip can be in user region. But then realized that what we
> are checking here is kernel address used in real mode. The use of
> REGION_ID eventhough simpler is confusing. Hence adding the comment with
> details Paul mentioned in email will help.
>
>
I've tried and covered it in the changelog, I thought a code comment
would make sense for the very non  obvious cases and not repeat what
the code does as comment

Balbir Singh.