NFSv4: Ensure the delegation cred is pinned when we call delegreturn

Ensure we don't release the delegation cred during the call to
nfs4_proc_delegreturn().

Fixes: ee05f456772d ("NFSv4: Fix races between open and delegreturn")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
index c17ff82..1865322 100644
--- a/fs/nfs/delegation.c
+++ b/fs/nfs/delegation.c
@@ -255,13 +255,18 @@
 
 static int nfs_do_return_delegation(struct inode *inode, struct nfs_delegation *delegation, int issync)
 {
+	const struct cred *cred;
 	int res = 0;
 
-	if (!test_bit(NFS_DELEGATION_REVOKED, &delegation->flags))
-		res = nfs4_proc_delegreturn(inode,
-				delegation->cred,
+	if (!test_bit(NFS_DELEGATION_REVOKED, &delegation->flags)) {
+		spin_lock(&delegation->lock);
+		cred = get_cred(delegation->cred);
+		spin_unlock(&delegation->lock);
+		res = nfs4_proc_delegreturn(inode, cred,
 				&delegation->stateid,
 				issync);
+		put_cred(cred);
+	}
 	return res;
 }