From f53e57975151f54ad8caa1b0ac8a78091cd5700a Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Mon, 19 Jan 2015 11:48:45 -0600
Subject: [PATCH] mnt: Fix the error check in __detach_mounts

lookup_mountpoint can return either NULL or an error value.
Update the test in __detach_mounts to test for an error value
to avoid pathological cases causing a NULL pointer dereferences.

The callers of __detach_mounts should prevent it from ever being
called on an unlinked dentry but don't take any chances.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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