nfs: move the nfs4_data_server_cache into struct nfs_net

Since struct nfs4_pnfs_ds should not be shared between net namespaces,
move from a global list of objects to a per-netns list and spinlock.

Tested-by: Sargun Dillon <sargun@sargun.me>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Link: https://lore.kernel.org/r/20250410-nfs-ds-netns-v2-2-f80b7979ba80@kernel.org


Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>