bcachefs: Fix compilation error for bch2_sb_member_alloc

bcachefs: Fix compilation error for bch2_sb_member_alloc

Fix the following compilation error:

```
fs/bcachefs/sb-members.c: In function ‘bch2_sb_member_alloc’:
fs/bcachefs/sb-members.c:508:2: error: a label can only be part of a statement and a declaration is not a statement
  508 |  unsigned nr_devices = max_t(unsigned, dev_idx + 1, c->sb.nr_devices);
```

Fixes: a7d364a133c7 ("bcachefs: bch2_sb_member_alloc()")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>