vdso/datapage: Use correct clock mode name in comment

While the explanation for time namespace <-> vdso interactions is very
helpful it uses the wrong name in the comment when describing the clock
mode making grepping a bit annoying.

This seems like an accidental oversight when moving from VCLOCK_TIMENS
to VDSO_CLOCKMODE_TIMENS. It seems that
660fd04f ("lib/vdso: Prepare for time namespace support") misspelled
VCLOCK_TIMENS as VLOCK_TIMENS which explains why it got missed when
VCLOCK_TIMENS became VDSO_CLOCKMODE_TIMENS in
2d6b01bd ("lib/vdso: Move VCLOCK_TIMENS to vdso_clock_modes").

Update the comment to use VDSO_CLOCKMODE_TIMENS.

Fixes: 660fd04f ("lib/vdso: Prepare for time namespace support")
Fixes: 2d6b01bd

 ("lib/vdso: Move VCLOCK_TIMENS to vdso_clock_modes")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andrei Vagin <avagin@gmail.com>
Acked-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/20200420100615.1549804-1-christian.brauner@ubuntu.com