bcachefs: remove write permission for gc_gens_pos sysfs interface

bcachefs: remove write permission for gc_gens_pos sysfs interface

The gc_gens_pos is used to show the status of bucket gen gc.
There is no need to assign write permissions for this attribute.
Here we can use read_attribute helper to define this attribute.

```
[Before]
  $ ll internal/gc_gens_pos
  -rw-r--r-- 1 root root 4096 Oct 28 15:27 internal/gc_gens_pos

[After]
  $ ll internal/gc_gens_pos
  -r--r--r-- 1 root root 4096 Oct 28 17:27 internal/gc_gens_pos
```

Fixes: ac516d0e7db7 ("bcachefs: Add the status of bucket gen gc to sysfs")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>