erofs: support idmapped mounts

This patch enables idmapped mounts for erofs, since all dedicated helpers
for this functionality existsm, so, in this patch we just pass down the
user_namespace argument from the VFS methods to the relevant helpers.

Simple idmap example on erofs image:

1. mkdir dir
2. touch dir/file
3. mkfs.erofs erofs.img dir
4. mount -t erofs -o loop erofs.img  /mnt/erofs/

5. ls -ln /mnt/erofs/
total 0
-rw-rw-r-- 1 1000 1000 0 May 17 15:26 file

6. mount-idmapped --map-mount b:1000:1001:1 /mnt/erofs/ /mnt/scratch_erofs/

7. ls -ln /mnt/scratch_erofs/
total 0
-rw-rw-r-- 1 1001 1001 0 May 17 15:26 file
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Chao Yu <chao.yu@oppo.com>
Link: https://lore.kernel.org/r/20220517104103.3570721-1-chao@kernel.orgSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/inode.c

@@ -365,7 +365,7 @@ int erofs_getattr(struct user_namespace *mnt_userns, const struct path *path,
 	stat->attributes_mask |= (STATX_ATTR_COMPRESSED |
 				  STATX_ATTR_IMMUTABLE);
 
-	generic_fillattr(&init_user_ns, inode, stat);
+	generic_fillattr(mnt_userns, inode, stat);
 	return 0;
 }
 

fs/erofs/super.c

@@ -821,7 +821,7 @@ static struct file_system_type erofs_fs_type = {
 	.name           = "erofs",
 	.init_fs_context = erofs_init_fs_context,
 	.kill_sb        = erofs_kill_sb,
-	.fs_flags       = FS_REQUIRES_DEV,
+	.fs_flags       = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("erofs");