NFSD: Convert the filecache to use rhashtable

NFSD: Convert the filecache to use rhashtable

[ Upstream commit ce502f81ba884c1fe45dc0ebddbcaaa4ec0fc5fb ]

Enable the filecache hash table to start small, then grow with the
workload. Smaller server deployments benefit because there should
be lower memory utilization. Larger server deployments should see
improved scaling with the number of open files.

Suggested-by: Jeff Layton <jlayton@kernel.org>
Suggested-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>