bcachefs: Fix potential sleeping during mount

During mount, bcachefs mount option processing may sleep while allocating a string buffer.

Fix this by reference counting in order to take the atomic path.

Signed-off-by: Daniel J Blueman <[e-mail]>
Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/disk_groups.c

···
 	case TARGET_DEV: {
 		struct bch_dev *ca;
 
+		out->atomic++;
 		rcu_read_lock();
 		ca = t.dev < c->sb.nr_devices
 			? rcu_dereference(c->devs[t.dev])
···
 		}
 
 		rcu_read_unlock();
+		out->atomic--;
 		break;
 	}
 	case TARGET_GROUP: