abd_iter_page: rework to handle multipage scatterlists

Previously, abd_iter_page() would assume that every scatterlist would
contain a single page (compound or no), because that's all we ever
create in abd_alloc_chunks(). However, scatterlists can contain multiple
pages of arbitrary provenance, and if we get one of those, we'd get all
the math wrong.

This reworks things to handle multiple pages in a scatterlist, by
properly finding the right page within it for the given offset, and
understanding better where the end of the page is and not crossing it.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reported-by: Brian Atkinson <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Brian Atkinson <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #16108 

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