lockd: Fix comment about NLMv3 backwards compatibility

NLMv2 is completely different protocol than NLMv1 and NLMv3, and in
original Sun implementation is used for RPC loopback callbacks from statd
to lockd services. Linux does not use nor does not implement NLMv2.

Hence, NLMv3 is not backward compatible with NLMv2. But NLMv3 is backward
compatible with NLMv1. Fix comment.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>