NFSD: Add struct nfsd4_fattr_args

I'm about to split nfsd4_encode_fattr() into a number of smaller
functions. Instead of passing a large number of arguments to each of
the smaller functions, create a struct that can gather the common
argument variables into something with a convenient handle on it.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>