NFS: Cleanup - only store the write verifier in struct nfs_page

The 'committed' field is not needed once we have put the struct nfs_page
on the right list.

Also correct the type of the verifier: it is not an array of __be32, but
simply an 8 byte long opaque array.

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