ocfs2: Clean up the checks for CoW and direct I/O.

When ocfs2 has to do CoW for refcounted extents, we disable direct I/O
and go through the buffered I/O path.  This makes the combined check
easier to read.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>