NFSv4: Support NFSv4 optional attributes in the struct nfs_fattr

Currently, filling struct nfs_fattr is more or less an all or nothing
operation, since NFSv2 and NFSv3 have only mandatory attributes.
In NFSv4, some attributes are optional, and so we may simply not be able to
fill in those fields. Furthermore, NFSv4 allows you to specify which
attributes you are interested in retrieving, thus permitting you to
optimise away retrieval of attributes that you know will no change...

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