CVE-2026-72362: drm/xe/pt: Fix NULL pointer dereference in xe_pt_zap_ptes_entry()

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

drm/xe/pt: Fix NULL pointer dereference in xeptzapptesentry()

The page-table walk framework may pass a NULL child pointer for unpopulated entries. xeptzapptesentry() called containerof(child) before checking for NULL, then dereferenced the result, causing a crash.

Move the containerof() call after a NULL guard, so the function returns early instead of proceeding with an invalid pointer. XEWARNON is kept to help root cause the issue, but we now bail instead of crashing the driver.

v2: Comment that triggering XEWARNON is unexpected behavior (Matt Brost)

(cherry picked from commit b9297d19d9df5d4b6c994648570c5dcd1cac68ff)