futex: Move futex cleanup to __mmdrop()

Futex hash allocations are done in mm_init() and the cleanup happens in
__mmput(). That works most of the time, but there are mm instances which
are instantiated via mm_alloc() and freed via mmdrop(), which causes the
futex hash to be leaked.

Move the cleanup to __mmdrop().

Fixes: 56180dd20c19 ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t")
Reported-by: André Draszik <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: André Draszik <[e-mail]>
Link: https://lore.kernel.org/all/87ldo5ihu0.ffs@tglx
Closes: https://lore.kernel.org/all/[e-mail]

kernel/fork.c

···
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
 	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
+	futex_hash_free(mm);
 
 	free_mm(mm);
 }
···
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
 	lru_gen_del_mm(mm);
-	futex_hash_free(mm);
 	mmdrop(mm);
 }