using the scalable vmapi API again

[Christoph Hellwig]

Hi Alex,

I've worked with Nick to make sure the regressions introduced by Nick's
scalable vmap API are fixed, so we an start using it again after we
had to disable it after a brief period in early 2009.

Can you make sure commits d2859751cd0bf586941ffa7308635a293f943c17 and
95f8e302c04c0b0c6de35ab399a5551605eeb006 get re-aplied to the XFS
tree, or maybe re-revert their revert commits
3a011a171906a3a51a43bb860fb7c66a64cab140 and
cf7dab801796b9ee52a6dc99888a66bf476538ec.