ptp: drain virtual clock sysfs operations before unregistering children

Parent clock removal walks its virtual clocks before removing the
n_vclocks sysfs attribute. The mutex taken by ptp_vclock_in_use() is
released before that walk, so a concurrent sysfs deletion can pick the
same child and unregister and free its ptp_vclock a second time. A
reference held by the device iterator does not protect that separately
allocated virtual clock.

Remove n_vclocks before walking the children. Removing the attribute
prevents new stores and drains stores already running, without holding
n_vclocks_mux across a callback that needs that mutex. Skip virtual
clocks, which do not have this attribute.

This race was found by code inspection of virtual-clock registration
failure cleanup and parent removal.

Fixes: bfcbb76b0f59 ("ptp: unregister virtual clocks when unregistering physical clock.")
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-v6-1-e705612541f1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>