This is finally the RAID5 Write support.

The bigger part of this patch is not the XOR engine itself, But the
read4write logic, which is a complete mini prepare_for_striping
reading engine that can read scattered pages of a stripe into cache
so it can be used for XOR calculation. That is, if the write was not
stripe aligned.

The main algorithm behind the XOR engine is the 2 dimensional array:
	struct __stripe_pages_2d.
A drawing might save 1000 words
---

__stripe_pages_2d
       |
 n = pages_in_stripe_unit;
 w = group_width - parity;
       |                            pages array presented to the XOR lib
       |                                                |
       V                                                |
 __1_page_stripe[0].pages --> [c0][c1]..[cw][c_par] <---|
       |                                                |
 __1_page_stripe[1].pages --> [c0][c1]..[cw][c_par] <---
       |
...    |                         ...
       |
 __1_page_stripe[n].pages --> [c0][c1]..[cw][c_par]
                               ^
                               |
           data added columns first then row

---
The pages are put on this array columns first. .i.e:
	p0-of-c0, p1-of-c0, ... pn-of-c0, p0-of-c1, ...
So we are doing a corner turn of the pages.

Note that pages will zigzag down and left. but are put sequentially
in growing order. So when the time comes to XOR the stripe, only the
beginning and end of the array need be checked. We scan the array
and any NULL spot will be field by pages-to-be-read.

The FS that wants to support RAID5 needs to supply an
operations-vector that searches a given page in cache, and specifies
if the page is uptodate or need reading. All these pages to be read
are put on a slave ore_io_state and synchronously read. All the pages
of a stripe are read in one IO, using the scatter gather mechanism.

In write we constrain our IO to only be incomplete on a single
stripe. Meaning either the complete IO is within a single stripe so
we might have pages to read from both beginning  or end of the
strip. Or we have some reading to do at beginning but end at strip
boundary. The left over pages are pushed to the next IO by the API
already established by previous work, where an IO offset/length
combination presented to the ORE might get the length truncated and
the user must re-submit the leftover pages. (Both exofs and NFS
support this)

But any ORE user should make it's best effort to align it's IO
before hand and avoid complications. A cached ore_layout->stripe_size
member can be used for that calculation. (NOTE: that ORE demands
that stripe_size may not be bigger then 32bit)

What else? Well read it and tell me.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>