md/raid5: let multiple devices of stripe_head share page

In current implementation, grow_buffers() uses alloc_page() to
allocate the buffers for each stripe_head, i.e. allocate a page
for each dev[i] in stripe_head.

After setting stripe_size as a configurable value by writing
sysfs entry, it means that we always allocate 64K buffers, but
just use 4K of them when stripe_size is 4K in 64KB arm64.

To avoid wasting memory, we try to let multiple sh->dev share
one real page. That means, multiple sh->dev[i].page will point
to the only page with different offset. Example of 64K PAGE_SIZE
and 4K stripe_size as following:

                    64K PAGE_SIZE
          +---+---+---+---+------------------------------+
          |   |   |   |   |
          |   |   |   |   |
          +-+-+-+-+-+-+-+-+------------------------------+
            ^   ^   ^   ^
            |   |   |   +----------------------------+
            |   |   |                                |
            |   |   +-------------------+            |
            |   |                       |            |
            |   +----------+            |            |
            |              |            |            |
            +-+            |            |            |
              |            |            |            |
        +-----+-----+------+-----+------+-----+------+------+
sh      | offset(0) | offset(4K) | offset(8K) | offset(12K) |
 +      +-----------+------------+------------+-------------+
 +----> dev[0].page  dev[1].page  dev[2].page  dev[3].page

A new 'pages' array will be added into stripe_head to record shared
page used by this stripe_head. Allocate them when grow_buffers()
and free them when shrink_buffers().

After trying to share page, the users of sh->dev[i].page need to take
care of the related page offset: page of issued bio and page passed
to xor compution functions. But thanks for previous different page offset
supported. Here, we just need to set correct dev[i].offset.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>