Patch "fs: use RCU read side protection in d_validate" broken

[Nick Piggin <npiggin@kernel.dk>]

This patch is totally broken. You can't just dget() a dentry with
nothing but RCU critical section open.

The patch in my tree this is claimed to be split out of, at least
attemptet to do some locking, I don't know why that was stripped
out. But I didn't get that quite right myself at which point I
decided to just forget about it entirely.

Christoph, why did you think such a patch is worth getting merged, btw?
I saw no hint of a justification in your changelog.  I mean, in my tree
at least there was a _rationale_ that dcache_lock is going away and this
marginally made the locking simpler. But it doesn't make sense in the
current tree, even if the merged patch was _not_ buggy -- what were you
trying to do, make ncpfs's readdir go really fast?

Breaking things out when they make sense and stand as patches on their
own. That is very important. When they do _not_ make sense on their own,
then that is when they do not make sense to be broken out and merged
ahead of their series.

Linus, I cooked up a fix for it, but decided it's stupid to bother with
any complexity here. Please revert
3825bdb7ed920845961f32f364454bee5f469abb

If I end up wanting it again for dcache series, I can do it with proper
locking and proper justification.

Thanks,
Nick