With write back cache, we use orig_page to do prexor. This patch
makes sure we read data into orig_page for it.

Flag R5_OrigPageUPTDODATE is added to show whether orig_page
has the latest data from raid disk.

We introduce a helper function uptodate_for_rmw() to simplify
the a couple conditions in handle_stripe_dirtying().
Signed-off-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NShaohua Li <shli@fb.com>