NILFS2: support NFSv2 export

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: NeilBrown <neilb@suse.de>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>