NFS: Remove nfs_begin_data_update/nfs_end_data_update

The lower level routines in fs/nfs/proc.c, fs/nfs/nfs3proc.c and
fs/nfs/nfs4proc.c should already be dealing with the revalidation issues.

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