NFSD: Report filecache LRU size

[ Upstream commit 0fd244c1 ]

Surface the NFSD filecache's LRU list length to help field
troubleshooters monitor filecache issues.

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