dax: Make sure the unlocking entry isn't locked

I wrote the semantics in the commit message, but didn't document it in
the source code.  Use a BUG_ON instead (if any code does do this, it's
really buggy; we can't recover and it's worth taking the machine down).

Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/dax.c

@@ -255,6 +255,7 @@ static void dax_unlock_entry(struct xa_state *xas, void *entry)
 {
 	void *old;
 
+	BUG_ON(dax_is_locked(entry));
 	xas_reset(xas);
 	xas_lock_irq(xas);
 	old = xas_store(xas, entry);