UPSTREAM: ext4 crypto: use dget_parent() in ext4_d_revalidate()

commit 3d43bcfe upstream.

This avoids potential problems caused by a race where the inode gets
renamed out from its parent directory and the parent directory is
deleted while ext4_d_revalidate() is running.

Fixes: 28b4c263
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 41948f88)
Change-Id: Icea8f440a708274ecfa12d0c2a392b6aa3cc22c4
Signed-off-by: Eric Biggers <ebiggers@google.com>