net: macsec: add nla support for changing the offloading selection

MACsec offloading to underlying hardware devices is disabled by default
(the software implementation is used). This patch adds support for
changing this setting through the MACsec netlink interface. Many checks
are done when enabling offloading on a given MACsec interface as there
are limitations (it must be supported by the hardware, only a single
interface can be offloaded on a given physical device at a time, rules
can't be moved for now).

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>