alternative take on the same page merging leak fix v2

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens, hi Ming,

this is the tested and split version of what I think is the better
fix for the get_user_pages page leak, as it leaves the intelligence
in the callers instead of in bio_try_to_merge_page.

Changes since v1:
 - drop patches not required for 5.2
 - added Reviewed-by tags