For some reason (I don't rememeber why), RX timestamps are retrieved
from the socket, while TX timestamps are with tops->get.

Thus, there was no utc/tai correction on receive stamps (i.e. T2).
This affects us when we are driven by a WR switch, that properly
differentiates between UTC and TAI.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>