vdso/gettimeofday: Clarify comment about open coded function

The two comments state, that the following code open codes something but
they lack to specify what exactly is open coded.

Expand comments by mentioning the reference to the open coded function.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20240701-vdso-cleanup-v1-1-36eb64e7ece2@linutronix.de