Update relatime comments to include equality

relatime also updates atime if the previous atime is equal to one or
both of the ctime and mtime; a non-strict interpretation of "earlier
than" and "younger than" in the comments allows this, but for clarity,
this makes it explicit.

Pointed out by "epiii2" and "ctrl-alt-delor" in
https://unix.stackexchange.com/q/740862/86440

.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>