[ Upstream commit 4ea899ead2786a30aaa8181fefa81a3df4ad28f6 ]

Introduce a local wait_for_completion variable to avoid an access to the
potentially freed dio struture after dropping the last reference count.

Also use the chance to document the completion behavior to make the
refcounting clear to the reader of the code.

Fixes: ff6a9292 ("iomap: implement direct I/O")
Reported-by: NChandan Rajendra <chandan@linux.ibm.com>
Reported-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NChandan Rajendra <chandan@linux.ibm.com>
Tested-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>