futex: Move futex_hash_free() back to __mmput()

futex: Move futex_hash_free() back to __mmput()

To avoid a memory leak via mm_alloc() + mmdrop() the futex cleanup code
has been moved to __mmdrop(). This resulted in a warnings if the futex
hash table has been allocated via vmalloc() the mmdrop() was invoked
from atomic context.
The free path must stay in __mmput() to ensure it is invoked from
preemptible context.

In order to avoid the memory leak, delay the allocation of
mm_struct::mm->futex_ref to futex_hash_allocate(). This works because
neither the per-CPU counter nor the private hash has been allocated and
therefore
- futex_private_hash() callers (such as exit_pi_state_list()) don't
  acquire reference if there is no private hash yet. There is also no
  reference put.

- Regular callers (futex_hash()) fallback to global hash. No reference
  counting here.

The futex_ref member can be allocated in futex_hash_allocate() before
the private hash itself is allocated. This happens either while the
first thread is created or on request. In both cases the process has
just a single thread so there can be either futex operation in progress
or the request to create a private hash.

Move futex_hash_free() back to __mmput();
Move the allocation of mm_struct::futex_ref to futex_hash_allocate().

  [ bp: Fold a follow-up fix to prevent a use-after-free:
    https://lore.kernel.org/r/20250830213806.sEKuuGSm@linutronix.de ]

Fixes:  e703b7e247503 ("futex: Move futex cleanup to __mmdrop()")
Closes: https://lore.kernel.org/all/20250821102721.6deae493@kernel.org/
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lkml.kernel.org/r/20250822141238.PfnkTjFb@linutronix.de