linux-imx

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>