ptp: vclock: preserve missing hardware timestamps

Socket timestamp delivery converts hardware timestamps to a bound
virtual clock before checking whether a hardware timestamp is present.
A zero timestamp means that the packet has no hardware timestamp, but
timecounter_cyc2time() can turn that zero into a nonzero virtual time.
The socket then reports a hardware timestamp which was never supplied.

Return zero without conversion when the input timestamp is absent.
This also covers skb_get_tx_timestamp(), which checks for an absent
timestamp after the same conversion.

This was identified during review of virtual clock sampling changes.

Fixes: d7c088265588 ("net: socket: support hardware timestamp conversion to PHC bound")
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Link: https://patch.msgid.link/20261026-ptp-vclock-sampling-v6-5-e705612541f1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>