iomap: move the iomap_dio_rw ->end_io callback into a structure

Add a new iomap_dio_ops structure that for now just contains the end_io
handler.  This avoid storing the function pointer in a mutable structure,
which is a possible exploit vector for kernel code execution, and prepares
for adding a submit_io handler that btrfs needs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 7441129..21bd3d5 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -442,6 +442,10 @@ xfs_dio_write_end_io(
 	return error;
 }
 
+static const struct iomap_dio_ops xfs_dio_write_ops = {
+	.end_io		= xfs_dio_write_end_io,
+};
+
 /*
  * xfs_file_dio_aio_write - handle direct IO writes
  *
@@ -542,7 +546,7 @@ xfs_file_dio_aio_write(
 	}
 
 	trace_xfs_file_direct_write(ip, count, iocb->ki_pos);
-	ret = iomap_dio_rw(iocb, from, &xfs_iomap_ops, xfs_dio_write_end_io);
+	ret = iomap_dio_rw(iocb, from, &xfs_iomap_ops, &xfs_dio_write_ops);
 
 	/*
 	 * If unaligned, this is the only IO in-flight. If it has not yet