NFS: Move and update xdr_decode_foo() functions that we're keeping

Clean up.

Move the timestamp decoder to match the placement and naming
conventions of the other helpers.  Fold xdr_decode_fattr() into
decode_fattr(), which is now it's only user.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>