NFS: Use parent's objective cred in nfs_access_login_time()

The subjective cred (task->cred) can potentially be overridden and
subsquently freed in non-RCU context, which could lead to a panic if we
try to use it in cred_fscmp().  Use __task_cred(), which returns the
objective cred (task->real_cred) instead.

Fixes: 0eb4381 ("NFS: Clear the file access cache upon login")
Fixes: 5e9a7b9 ("NFS: Fix up a sparse warning")

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/dir.c

@@ -2963,7 +2963,7 @@ static u64 nfs_access_login_time(const struct task_struct *task,
 	rcu_read_lock();
 	for (;;) {
 		parent = rcu_dereference(task->real_parent);
-		pcred = rcu_dereference(parent->cred);
+		pcred = __task_cred(parent);
 		if (parent == task || cred_fscmp(pcred, cred) != 0)
 			break;
 		task = parent;