2.6.17: lockless pagecache

Updated lockless pagecache patchset available here:

ftp://ftp.kernel.org/pub/linux/kernel/people/npiggin/patc...

This should hopefully be my last release using the old (2.6.17)
indirect radix-tree, and I'll switch to the direct radix-tree in
future.

Changes since last release:
- lots of radix-tree cleanups and bugs fixed
- radix-tree tag lookups may be lockless
- added some missing memory barriers
- lockless pagevec_lookup_tag

The last item allowed me to remove the last few read-lockers,
which is nice.