acquiring duplicate lock of same type: "vnode interlock"

[Don Lewis]

On  8 Dec, Jun Kuriyama wrote:
> 
> I got a witness message in my console with today's -current (not RELENG_5_2).
> 
> 
> ----- console
> acquiring duplicate lock of same type: "vnode interlock"
>  1st vnode interlock @ ufs/ffs/ffs_snapshot.c:1794
>  2nd vnode interlock @ ufs/ffs/ffs_snapshot.c:1796
> Stack backtrace:
> backtrace(c0734d9b,c073959e,c0748dca,704,c82a0008) at backtrace+0x17
> witness_lock(c864c6f0,8,c0748dca,704,c860bdac) at witness_lock+0x672
> _mtx_lock_flags(c864c6f0,0,c0748dc1,704,c072bed0) at _mtx_lock_flags+0xba
> ffs_snapshot_mount(c85b2800,c85b2800,c8599000,c42f1600,c42f1600) at ffs_snapshot_mount+0x2b0
> ffs_mountfs(c864ccb8,c85b2800,c82a8000,c0557bd0,c07317d6) at ffs_mountfs+0xa6e

I think this is probably harmless, but it I think it should be fixed.
It's a bit tricky since the locking in ffs_snapshot_mount() has to be
compatible with the locking in ffs_snapshot_unmount().