ptp: ptp_vmclock: return TAI not UTC

ptp: ptp_vmclock: return TAI not UTC

To output UTC would involve complex calculations about whether the time
elapsed since the reference time has crossed the end of the month when
a leap second takes effect. I've prototyped that, but it made me sad.

Much better to report TAI, which is what PHCs should do anyway.
And much much simpler.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Babis Chalios <bchalios@amazon.es>
Tested-by: Takahiro Itazuri <itazur@amazon.com>
Link: https://patch.msgid.link/20260130173704.12575-8-itazur@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>