ioctl: handle idmapped mounts

Enable generic ioctls to handle idmapped mounts by passing down the
mount's user namespace. If the initial user namespace is passed nothing
changes so non-idmapped mounts will see identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-22-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/remap_range.c

@@ -432,13 +432,16 @@ EXPORT_SYMBOL(vfs_clone_file_range);
 /* Check whether we are allowed to dedupe the destination file */
 static bool allow_file_dedupe(struct file *file)
 {
+	struct user_namespace *mnt_userns = file_mnt_user_ns(file);
+	struct inode *inode = file_inode(file);
+
 	if (capable(CAP_SYS_ADMIN))
 		return true;
 	if (file->f_mode & FMODE_WRITE)
 		return true;
-	if (uid_eq(current_fsuid(), file_inode(file)->i_uid))
+	if (uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)))
 		return true;
-	if (!inode_permission(&init_user_ns, file_inode(file), MAY_WRITE))
+	if (!inode_permission(mnt_userns, inode, MAY_WRITE))
 		return true;
 	return false;
 }