nfsd: set missing after_change as before_change + 1

nfsd: set missing after_change as before_change + 1

In the event that we can't fetch post_op_attr attributes, we still need
to set a value for the after_change. The operation has already happened,
so we're not able to return an error at that point, but we do want to
ensure that the client knows that its cache should be invalidated.

If we weren't able to fetch post-op attrs, then just set the
after_change to before_change + 1. The atomic flag should already be
clear in this case.

Suggested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>