mbuf: add Rx timestamp flag and helpers

mbuf: add Rx timestamp flag and helpers

There is already a dynamic field for timestamp,
used only for Tx scheduling with the dedicated Tx offload flag.
The same field can be used for Rx timestamp filled by drivers.

A new dynamic flag is defined for Rx usage.
A new function wraps the registration of both field and Rx flag.
The type rte_mbuf_timestamp_t is defined for the API users.

After migrating all Rx timestamp usages, it will be possible
to remove the deprecated timestamp field.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Olivier Matz <olivier.matz@6wind.com>