kern/39371: wapbl should allow mounting "/" even if journal is hosed

[tnn%NetBSD.org@localhost]

>Number:         39371
>Category:       kern
>Synopsis:       wapbl should allow mounting "/" even if journal is hosed
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Aug 17 20:00:01 +0000 2008
>Originator:     Tobias Nygren
>Release:        4.99.72
>Organization:
>Environment:
NetBSD x40.int.nygren.pp.se 4.99.72 NetBSD 4.99.72 (GENERIC.x40) #1: Tue Aug 12 
12:40:16 CEST 2008  
tnn%x40.int.nygren.pp.se@localhost:/usb/obj/sys/arch/i386/compile/GENERIC.x40 
i386
>Description:
The journal of my root filesystem became corrupt for some reason, leaving me 
with no sane recovery method:

boot device: wd0
root on wd0a dumps on wd0b
Unrecognized wapbl type: 0x08012180
no file system for wd0 (dev 0x0)
cannot mount root, error = 79
root device (default wd0a): ddb

I had to use ddb to trick the kernel into mounting the root fs without the 
FS_DOWAPBL flag, after which I could run fsck(8) to recover.

I think we should allow mounting "/" read-only, even if log replay fails.

>How-To-Repeat:

>Fix: