NFSD: Report average age of filecache items

[ Upstream commit 904940e9 ]

This is a measure of how long items stay in the filecache, to help
assess how efficient the cache is.

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