LOR: ffs_snapshot.c from 5.2-BETA

[Don Lewis]

On  9 Dec, Jesse Guardiani wrote:
> Howdy list,
> 
> Has anyone seen this yet? Happened while I was fsck'ing my
> external USB hard drive a few days ago. Didn't see anything
> recent like this in the archive (Nov was the closest, but it
> looked like a separate issue).
> 
> 
> acquiring duplicate lock of same type: "vnode interlock"
>  1st vnode interlock @ ufs/ffs/ffs_snapshot.c:488
>  2nd vnode interlock @ ufs/ffs/ffs_snapshot.c:491
> Stack backtrace:
> backtrace(c06df4ef,c06e3b95,c06eca1f,1eb,246) at backtrace+0x17
> witness_lock(c42b9a28,8,c06eca1f,1eb,c3b48678) at witness_lock+0x672
> _mtx_lock_flags(c42b9a28,0,c06eca16,1eb,c19dbf00) at _mtx_lock_flags+0xba
> ffs_snapshot(c3abbc00,bfbfed88,70,c05482f3,c073f800) at ffs_snapshot+0x1759

A related LOR was reported a few days ago.  I think it is harmless, but
I also think I know how to fix it.