From: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Subject: fix write synchronization for DAX
Date: Tue, 10 Jan 2017 16:48:06 +0100
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While I've fixed both ext4 and XFS to not incorrectly allow parallel
writers when mounting with -o dax ext4 still has this issue after the
iomap conversion.

Patch 1 fixes it, and patch 2 adds a lockdep assert to catch any new
file systems copy and pasting from the direct I/O path.