Re: [md PATCH 12/36] md/raid1: store behind-write pages in bi_vecs.

NeilBrown [off-list ref] writes:

When performing write-behind we allocate pages to store the data
during write.
Previously we just keep a list of pages.  Now we keep a list of
bi_vec which includes offset and size.
This means that the r1bio has complete information to create a new
bio which will be needed for retrying after write errors.

Signed-off-by: NeilBrown <redacted>

Reviewed-by: Namhyung Kim <redacted>