LFS stability: kern/36608

Hi,

With the recent discussions on LFS stability, I'd like to point out PR
kern/36608 -- which I still can trigger more or less at will on today's
current.

From my (very incomplete) understanding of the LFS code, it looks like
lfs_vnops.c locks a vnode (simple lock) and then calls ltsleep on an lfs
struct.   Then, before ltsleep returns, something else (exactly what seems to
vary) comes along and locks the same vnode.  This looks to me suspiciously
like a locking bug.

I can "fix" the bug by (1) removing the call to lock the vnode or by (2)
turning off LOCKDEBUG.  I suspect, however, that either solution simply masks
the problem.  Incidentally, implementing (1) reverts the code to what it
looked like before mid April.

Sverre