macsec: add support for specifying offload at link add time

macsec: add support for specifying offload at link add time

This patch adds support for configuring offload mode upon MACsec
device creation.

If offload mode is not specified, then netlink attribute is not
added. Default behavior on the kernel side in this case is
backward-compatible (offloading is disabled by default).

Example:
$ ip link add link eth0 macsec0 type macsec port 11 encrypt on offload mac

Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David Ahern <dsahern@gmail.com>