nfsd: notify_change needs elevated write count

Looks like this bug has been here since these write counts were
introduced, not sure why it was just noticed now.

Thanks also to Jan Kara for pointing out the problem.

Cc: stable@vger.kernel.org
Reported-by: Matthew Rahtz <mrahtz@rapitasystems.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>