NFSv4.2: Fix a potential double free with READ_PLUS

kfree()-ing the scratch page isn't enough, we also need to set the pointer
back to NULL to avoid a double-free in the case of a resend.

Fixes: fbd2a05f (NFSv4.2: Rework scratch handling for READ_PLUS)
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4proc.c

@@ -5437,10 +5437,18 @@ static bool nfs4_read_plus_not_supported(struct rpc_task *task,
 	return false;
 }
 
-static int nfs4_read_done(struct rpc_task *task, struct nfs_pgio_header *hdr)
+static inline void nfs4_read_plus_scratch_free(struct nfs_pgio_header *hdr)
 {
-	if (hdr->res.scratch)
+	if (hdr->res.scratch) {
 		kfree(hdr->res.scratch);
+		hdr->res.scratch = NULL;
+	}
+}
+
+static int nfs4_read_done(struct rpc_task *task, struct nfs_pgio_header *hdr)
+{
+	nfs4_read_plus_scratch_free(hdr);
+
 	if (!nfs4_sequence_done(task, &hdr->res.seq_res))
 		return -EAGAIN;
 	if (nfs4_read_stateid_changed(task, &hdr->args))