Add radix-tree tagging so we can look up dirty or writeback pages in
O(log64(n)) time.

Each radix-tree node gains two bits for each slot: one for page dirtiness and
one for page writebackness.

If a tag bit is set on a leaf node, it indicates that item at the
corresponding slot is tagged (say, a dirty page).

If a tag bit is set in a non-leaf node it indicates that the same tag bit is
set in the subtree which lies under the corresponding slot.  ie: "there is a
dirty page under here somewhere, but you need to search down further to find
it".

A gang lookup function is provided which can walk the radix tree in
logarithmic time looking for items which are tagged, starting from a
specified offset.  We use this for in-order searches for dirty or writeback
pages.

There is a userspace test harness for this code at

http://www.zip.com.au/~akpm/linux/patches/stuff/rtth.tar.gz