bpf: Use refcount_t instead of atomic_t for mmap_count

Use an API that resembles more the actual use of mmap_count.

Found by cocci:
kernel/bpf/arena.c:245:6-25: WARNING: atomic_dec_and_test variation before object free at line 249.

Fixes: b90d77e ("bpf: Fix remap of arena.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412292037.LXlYSHKl-lkp@intel.com/
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Link: https://lore.kernel.org/r/6ecce439a6bc81adb85d5080908ea8959b792a50.1735542814.git.xiaopei01@kylinos.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Pei Xiao

kernel/bpf/arena.c

@@ -218,7 +218,7 @@ static u64 arena_map_mem_usage(const struct bpf_map *map)
 struct vma_list {
 	struct vm_area_struct *vma;
 	struct list_head head;
-	atomic_t mmap_count;
+	refcount_t mmap_count;
 };
 
 static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
@@ -228,7 +228,7 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
 	vml = kmalloc(sizeof(*vml), GFP_KERNEL);
 	if (!vml)
 		return -ENOMEM;
-	atomic_set(&vml->mmap_count, 1);
+	refcount_set(&vml->mmap_count, 1);
 	vma->vm_private_data = vml;
 	vml->vma = vma;
 	list_add(&vml->head, &arena->vma_list);
@@ -239,7 +239,7 @@ static void arena_vm_open(struct vm_area_struct *vma)
 {
 	struct vma_list *vml = vma->vm_private_data;
 
-	atomic_inc(&vml->mmap_count);
+	refcount_inc(&vml->mmap_count);
 }
 
 static void arena_vm_close(struct vm_area_struct *vma)
@@ -248,7 +248,7 @@ static void arena_vm_close(struct vm_area_struct *vma)
 	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
 	struct vma_list *vml = vma->vm_private_data;
 
-	if (!atomic_dec_and_test(&vml->mmap_count))
+	if (!refcount_dec_and_test(&vml->mmap_count))
 		return;
 	guard(mutex)(&arena->lock);
 	/* update link list under lock */