NFSv4.1/flexfiles: Add refcounting to struct nfs4_ff_layout_mirror

We do want to share mirrors between layout segments, so add a refcount
to enable that.

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

fs/nfs/flexfilelayout/flexfilelayout.h

@@ -77,6 +77,7 @@ struct nfs4_ff_layout_mirror {
 	u32				uid;
 	u32				gid;
 	struct rpc_cred			*cred;
+	atomic_t			ref;
 	spinlock_t			lock;
 	struct nfs4_ff_layoutstat	read_stat;
 	struct nfs4_ff_layoutstat	write_stat;