CVE-2026-89805: 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

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

1. The higher-order vmaallocfolio()/folioalloc() calls did not pass GFPNOWARN, 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 GFPNOWARN 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 MIGRATEPFNCOMPOUND unset for those PFNs.

2. In the freepages error path, order was computed via folioorder(pagefolio(page)) after putpage(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 ->copytoram() 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 drmpagemapaddr. Once the destination has been demoted to order-0 folios the drmpagemapaddr 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 migratevmapages() / migratedevicepages(), so its order cannot be used to detect the demotion - test the destination for MIGRATEPFNCOMPOUND instead. Factor the array population out into drmpagemapmigratepopulatesrcpages() and use it from both drmpagemapevicttoram() and drmpagemapmigratetoram().