NFS: Cleanup of the nfs_pageio code in preparation for a pnfs bugfix

We need to ensure that the layouts are set up before we can decide to
coalesce requests. To do so, we want to further split up the struct
nfs_pageio_descriptor operations into an initialisation callback, a
coalescing test callback, and a 'do i/o' callback.

This patch cleans up the existing callback methods before adding the
'initialisation' callback.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>