macsec: add support for IFLA_MACSEC_OFFLOAD in macsec_changelink

Add support for changing Macsec offload selection through the
netlink layer by implementing the relevant changes in
macsec_changelink.

Since the handling in macsec_changelink is similar to macsec_upd_offload,
update macsec_upd_offload to use a common helper function to avoid
duplication.

Example for setting offload for a macsec device:
    ip link set macsec0 type macsec offload mac

Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>