bcachefs: support idmap mounts

bcachefs: support idmap mounts

We enable idmapped mounts for bcachefs. Here, we just pass down
the user_namespace argument from the VFS methods to the relevant
helpers.

The idmap test in bcachefs is as following:

```
1. losetup /dev/loop1 bcachefs.img
2. ./bcachefs format /dev/loop1
3. mount -t bcachefs /dev/loop1 /mnt/bcachefs/
4. ./mount-idmapped --map-mount b:0:1000:1 /mnt/bcachefs /mnt/idmapped1/

ll /mnt/bcachefs
total 2
drwx------. 2 root root    0 Jun 14 14:10 lost+found
-rw-r--r--. 1 root root 1945 Jun 14 14:12 profile

ll /mnt/idmapped1/

total 2
drwx------. 2 1000 1000    0 Jun 14 14:10 lost+found
-rw-r--r--. 1 1000 1000 1945 Jun 14 14:12 profile

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>