lib/scatterlist: Fix to calculate the last_pg properly

lib/scatterlist: Fix to calculate the last_pg properly

The last_pg is wrong, it is actually the first page of the last
scatterlist element. To get the last page of the last scatterlist element
we have to add prv->length. So it is checking mergability against the
wrong page, Further, a SG element is not guaranteed to end on a page
boundary, so we have to check the sub page location also for merge
eligibility.

Fix the above by checking physical contiguity based on PFNs, compute the
actual last page and then call pages_are_mergable().

Fixes: 1567b49d1a40 ("lib/scatterlist: add check when merging zone device pages")
Link: https://lore.kernel.org/r/20230111101054.188136-1-yishaih@nvidia.com
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>