radix-tree pagecache for 2.4.19-pre4-ac4

Hi.

I've remade my patch for the latest -ac release. It is the radix-tree pagecache with the enhancements recommended by Andrew Morton. It should apply cleanly to 2.4.19-pre4-ac4.

Enjoy!

Art Haas

They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety.
 -- Benjamin Franklin, Historical Review of Pennsylvania, 1759