nfsd: remove the pages_flushed statistic from filecache

We're counting mapping->nrpages, but not all of those are necessarily
dirty. We don't really have a simple way to count just the dirty pages,
so just remove this stat since it's not accurate.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>