This adds initial support for NFS direct I/O in the 2.5 kernel.  many
have asked for this support to be included in 2.5.  this patch does not
provide working NFS direct I/O, but i'm sending what i have now so that
it can be included before October 20.

NFS direct I/O is enabled by its very own kernel config option.  when
enabled, the NFS client won't build to prevent people from using this and
possibly corrupting their NFS files.  later i will send a patch that
finishes the implementation.

[ Config option currently disabled ]