macsec: add support for changing the offloading mode

macsec: add support for changing the offloading mode

MacSEC can now be offloaded to specialized hardware devices. Offloading
is off by default when creating a new MACsec interface, but the mode can
be updated at runtime. This patch adds a new subcommand,
`ip macsec offload`, to allow users to select the offloading mode of a
MACsec interface. It takes the mode to switch to as an argument, which
can for now either be 'off' or 'phy':

  # ip macsec offload macsec0 phy
  # ip macsec offload macsec0 off

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David Ahern <dsahern@gmail.com>