statx: Update offset commentary for struct statx

In commit 2a82bb02

 ("statx: stx_subvol"), a new member was added to
struct statx, but the offset comment was not correct. Update it.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240529081725.3769290-1-john.g.garry@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>