ptp: vclock: serialize extended samples with counter updates

The extended and cross-timestamp callbacks sample the physical clock
before taking the virtual clock mutex. A concurrent frequency adjustment
can advance the timecounter and change its multiplier in between, so the
older sample is converted using a different frequency than the one in
effect when it was taken.

Take the mutex before sampling and keep it held through the conversion.
Retain timecounter_cyc2time() and its existing conversion window. A PHC
sample need not advance the counter: for example, e1000e explicitly
allows non-monotonic SYSTIM readings. Treating a slightly older sample
as a forward delta would instead advance virtual time by nearly a full
counter wrap.

This race was found by code inspection of the virtual clock sampling
paths.

Fixes: f0067ebfc42b ("ptp: add gettimex64() to virtual clocks.")
Fixes: 21fad63084c2 ("ptp: add getcrosststamp() to virtual clocks.")
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Link: https://patch.msgid.link/20261026-ptp-vclock-sampling-v6-4-e705612541f1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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