Move as much as possible to kernel space; leave the parts in user space
that rely on checking memory allocation failures to detect the
transition between an exceptional entry and a bitmap.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Convert to new API and move to kernel space. Take the opportunity to
test the situation a little more thoroughly (ie at different offsets).
Signed-off-by: Matthew Wilcox <willy@infradead.org>