mmgrab() reference on the DF shim mm_struct is a no-op (mm_count is never read) and dev_priv->mm_structs is keyed by the recyclable current->mm pointer: i915_mm_struct.mm dangles after process exit and can silently alias a later process

Summary

mmgrab increments the shim's mm_count which nothing ever reads; the shim lifetime is governed by mm->refs, so the intended reference is never taken (the pairing mmdrop is #if 0'd). Wrappers dangle after process exit and the hash lookup by pointer can silently alias a recycled shim mm of a new process. Latent today: CONFIG_MMU_NOTIFIER is undefined tree-wide so every live deref is compiled out and unpriv creates fail at the stub. Make-mmgrab-real or key-by-proc fix.