drm/pagemap: Fix folio allocation fallback and use-after-put

In the Linux kernel, the following vulnerability has been resolved:

drm/pagemap: Fix folio allocation fallback and use-after-put

drm_pagemap_migrate_populate_ram_pfn() had two issues when populating
RAM PFNs with higher-order folios:

1. The higher-order vma_alloc_folio()/folio_alloc() calls did not pass
__GFP_NOWARN, so a THP allocation failure under memory pressure
would spam the kernel log, and there was no fallback path despite a
TODO comment stating one was needed. Add __GFP_NOWARN to the
higher-order allocation and, on failure, fall back to order-0
allocations for the entire range originally covered by the failed
higher-order allocation, leaving MIGRATE_PFN_COMPOUND unset for
those PFNs.

2. In the free_pages error path, order was computed via
folio_order(page_folio(page)) *after* put_page(page) had already
dropped the reference, resulting in a use-after-free/put when that
was the last reference on the page. Compute order before releasing
the page.

Introducing the fallback in 1. also requires the source page array
handed to ->copy_to_ram() to be built differently. Both callers only
populated the entry at the head of each source folio, relying on the
copy callback to derive the rest of the folio from the order recorded
in the matching drm_pagemap_addr. Once the destination has been demoted
to order-0 folios the drm_pagemap_addr entries are per-page, so a source
page is needed for every one of them; leaving them NULL makes the copy
callback stop after the first page and the remainder of the range is
never copied.

The source folio is only split later, by migrate_vma_pages() /
migrate_device_pages(), so its order cannot be used to detect the
demotion - test the destination for MIGRATE_PFN_COMPOUND instead. Factor
the array population out into drm_pagemap_migrate_populate_src_pages()
and use it from both drm_pagemap_evict_to_ram() and
__drm_pagemap_migrate_to_ram().