The "fh_len" passed to ->fh_to_* is not guaranteed to be that same as
that returned by encode_fh - it may be larger.

With NFSv2, the filehandle is fixed length, so it may appear longer
than expected and be zero-padded.

So we must test that fh_len is at least some value, not exactly equal
to it.
Signed-off-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NJan Kara <jack@suse.cz>