lockdep: Fix block chain corruption

Kent reported an occasional KASAN splat in lockdep. Mark then noted:

> I suspect the dodgy access is to chain_block_buckets[-1], which hits the last 4
> bytes of the redzone and gets (incorrectly/misleadingly) attributed to
> nr_large_chain_blocks.

That would mean @SiZe == 0, at which point size_to_bucket() returns -1
and the above happens.

alloc_chain_hlocks() has 'size - req', for the first with the
precondition 'size >= rq', which allows the 0.

This code is trying to split a block, del_chain_block() takes what we
need, and add_chain_block() puts back the remainder, except in the
above case the remainder is 0 sized and things go sideways.

Fixes: 810507f ("locking/lockdep: Reuse freed chain_hlocks entries")
Reported-by: Kent Overstreet <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Kent Overstreet <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

kernel/locking/lockdep.c

@@ -3497,7 +3497,8 @@ static int alloc_chain_hlocks(int req)
 		size = chain_block_size(curr);
 		if (likely(size >= req)) {
 			del_chain_block(0, size, chain_block_next(curr));
-			add_chain_block(curr + req, size - req);
+			if (size > req)
+				add_chain_block(curr + req, size - req);
 			return curr;
 		}
 	}