net: macsec: indicate next pn update when offloading

Indicate next PN update using update_pn flag in macsec_context.
Offloaded MACsec implementations does not know whether or not the
MACSEC_SA_ATTR_PN attribute was passed for an SA update and assume
that next PN should always updated, but this is not always true.

The PN can be reset to its initial value using the following command:
$ ip macsec set macsec0 tx sa 0 off #octeontx2-pf case

Or, the update PN command will succeed even if the driver does not support
PN updates.
$ ip macsec set macsec0 tx sa 0 pn 1 on #mscc phy driver case

Comparing the initial PN with the new PN value is not a solution. When
the user updates the PN using its initial value the command will
succeed, even if the driver does not support it. Like this:
$ ip macsec add macsec0 tx sa 0 pn 1 on key 00 \
ead3664f508eb06c40ac7104cdae4ce5
$ ip macsec set macsec0 tx sa 0 pn 1 on #mlx5 case

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>