binder: fix double-free in dbitmap

A process might fail to allocate a new bitmap when trying to expand its
proc->dmap. In that case, dbitmap_grow() fails and frees the old bitmap
via dbitmap_free(). However, the driver calls dbitmap_free() again when
the same process terminates, leading to a double-free error:

  ==================================================================
  BUG: KASAN: double-free in binder_proc_dec_tmpref+0x2e0/0x55c
  Free of addr ffff00000b7c1420 by task kworker/9:1/209

  CPU: 9 UID: 0 PID: 209 Comm: kworker/9:1 Not tainted 6.17.0-rc6-dirty #5 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  Workqueue: events binder_deferred_func
  Call trace:
   kfree+0x164/0x31c
   binder_proc_dec_tmpref+0x2e0/0x55c
   binder_deferred_func+0xc24/0x1120
   process_one_work+0x520/0xba4
  [...]

  Allocated by task 448:
   __kmalloc_noprof+0x178/0x3c0
   bitmap_zalloc+0x24/0x30
   binder_open+0x14c/0xc10
  [...]

  Freed by task 449:
   kfree+0x184/0x31c
   binder_inc_ref_for_node+0xb44/0xe44
   binder_transaction+0x29b4/0x7fbc
   binder_thread_write+0x1708/0x442c
   binder_ioctl+0x1b50/0x2900
  [...]
  ==================================================================

Fix this issue by marking proc->map NULL in dbitmap_free().

Cc: stable@vger.kernel.org
Fixes: 15d9da3 ("binder: use bitmap for faster descriptor lookup")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Tiffany Yang <ynaffit@google.com>
Link: https://lore.kernel.org/r/20250915221248.3470154-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Carlos Llamas

drivers/android/dbitmap.h

@@ -37,6 +37,7 @@ static inline void dbitmap_free(struct dbitmap *dmap)
 {
 	dmap->nbits = 0;
 	kfree(dmap->map);
+	dmap->map = NULL;
 }
 
 /* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */