btrfs: allow idmapped setattr inode op

Enable btrfs_setattr() to handle idmapped mounts. This is just a matter
of passing down the mount's userns.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5392,7 +5392,7 @@ static int btrfs_setattr(struct user_namespace *mnt_userns, struct dentry *dentr
 	if (btrfs_root_readonly(root))
 		return -EROFS;
 
-	err = setattr_prepare(&init_user_ns, dentry, attr);
+	err = setattr_prepare(mnt_userns, dentry, attr);
 	if (err)
 		return err;
 
@@ -5403,13 +5403,12 @@ static int btrfs_setattr(struct user_namespace *mnt_userns, struct dentry *dentr
 	}
 
 	if (attr->ia_valid) {
-		setattr_copy(&init_user_ns, inode, attr);
+		setattr_copy(mnt_userns, inode, attr);
 		inode_inc_iversion(inode);
 		err = btrfs_dirty_inode(inode);
 
 		if (!err && attr->ia_valid & ATTR_MODE)
-			err = posix_acl_chmod(&init_user_ns, inode,
-					      inode->i_mode);
+			err = posix_acl_chmod(mnt_userns, inode, inode->i_mode);
 	}
 
 	return err;