NFS4ERR_UNION_NOTSUPP is not a valid error for READ_PLUS

The client doesn't tell the server what data_content4 to read, but the
server determines it based on the offset.  Thus returning
NFS4ERR_UNION_NOTSUPP for READ_PLUS does not make sense.

Signed-off-by: loghyr <loghyr@gmail.com>
Merge branch 'master' of https://github.com/loghyr/NFSv4.2