ptp: Speed up vclock lookup

ptp_convert_timestamp() is called in the RX path of network messages.
The current implementation takes ~5000ns on 1.2GHz A53. This is too much
for the hot path of packet processing.

Introduce hash table for fast vclock lookup in ptp_convert_timestamp().
The execution time of ptp_convert_timestamp() is reduced to ~700ns on
1.2GHz A53.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>