objrmap doesn't work for nonlinear VMAs because the assumption that
offset-into-file correlates with offset-into-virtual-addresses does not
hold. Hence what try_to_unmap_cluster does is a mini "virtual scan" of
each nonlinear VMA which maps the file to which the target page belongs.
If vma locked, mlock the pages in the cluster, rather than unmapping them.
However, not all pages are guarantee page locked instead of the check
page, resulting in the below BUG_ON().
It's safe to mlock_vma_page() without PageLocked, so fix this issue by
removing that BUG_ON().