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This allows the direct I/O path to do I/O and raise & lower page->_refcount
while we're executing a truncate/hole punch.  This leads to us trying to free
a page with an elevated refcount.

I don't see how this is possible in XFS - maybe I'm missing
something, but "direct IO submission during truncate" is not
something that should ever be happening in XFS, DAX or not.

The pages involved in a direct I/O are not that of the file that
the direct I/O read/write syscalls are called on, but those of the
memory regions the direct I/O read/write syscalls operate on.
Those pages could be file backed and undergo a truncate at the
same time.