fix write synchronization for DAX

Christoph Hellwig
2017-01-10T15:48:06Z

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.