Pagetable teardown can hold page_table_lock for extremely long periods -
hundreds of milliseconds.  This is pretty much the final source of high
scheduling latency in the core kernel.

We fixed it for zap_page_range() by chunking the work up and dropping the
lock occasionally if needed.  But that did not fix exit_mmap() and
unmap_region().

So what this patch does is to create an uber-zapper "unmap_vmas()" which
provides all the vma-walking, page unmapping and low-latency lock-dropping
which zap_page_range(), exit_mmap() and unmap_region() require.  Those three
functions are updated to call unmap_vmas().

It's actually a bit of a cleanup...