macsec: report the offloading mode currently selected

macsec: report the offloading mode currently selected

This patch adds support to report the MACsec offloading mode currently
being enabled, which as of now can either be 'off' or 'phy'. This
information is reported through the `ip macsec show` command:

  # ip macsec show
  18: macsec0: protect on validate strict sc off sa off encrypt on send_sci on end_station off scb off replay off
      cipher suite: GCM-AES-128, using ICV length 16
      TXSC: 3e5035b67c860001 on SA 0
          0: PN 1, state on, key 00000000000000000000000000000000
      RXSC: b4969112700f0001, state on
          0: PN 1, state on, key 01000000000000000000000000000000
      offload: phy
  19: macsec1: protect on validate strict sc off sa off encrypt on send_sci on end_station off scb off replay off
      cipher suite: GCM-AES-128, using ICV length 16
      TXSC: 3e5035b67c880001 on SA 0
          1: PN 1, state on, key 00000000000000000000000000000000
      RXSC: b4969112700f0001, state on
          1: PN 1, state on, key 01000000000000000000000000000000
      offload: off

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