NFSv4: Translate NFS4ERR_BADNAME into ENOENT when applied to a lookup

Both LOOKUP and OPEN operations may return NFS4ERR_BADNAME if we send a
an invalid name as a filename argument. As far as the application is
concerned, it just has to know that the file doesn't exist, and so
ENOENT would be the appropriate reply. We should only return EINVAL
if the filename is being used to _create_ a new object on the
remote filesystem.

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