uio: fix finding mm index for vma

When finding mm index for vma it looks more flexible that the mm could
be sparse, and both the size of mm and the pgoff of vma could give
correct selection.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Hans J. Koch <hjk@hansjkoch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>