kern/52510: lfs_update_single does lfs_dino_setblocks without holding vn_lock

[coypu%sdf.org@localhost]

>Number:         52510
>Category:       kern
>Synopsis:       lfs_update_single does lfs_dino_setblocks without holding vn_lock
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Aug 28 11:25:00 +0000 2017
>Originator:     coypu
>Release:        NetBSD 8.99.1
>Organization:
>Environment:
NetBSD localhost 8.99.1 NetBSD 8.99.1 (GENERIC) #0: Fri Aug 25 00:07:03 UTC 2017  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
This is the actual cause of the asserts mentioned in kern/52127:

System panicked: kernel diagnostic assertion "(oip->i_size != 0 || lfs_dino_getblocks(fs, oip->i_din) == 0)" failed: file "/usr/src/sys/ufs/lfs/lfs_inode.c", line 595 truncate to 0 but 31 blks/31 effblks

Reverting the hack produces these problems more often, especially while running firefox on LFS.
>How-To-Repeat:

>Fix:
WIP...