macsec: add Extended Packet Number support

macsec: add Extended Packet Number support

This patch adds support for extended packet number (XPN).
XPN can be configured by passing 'cipher gcm-aes-xpn-128' as part of
the ip link add command using macsec type.
In addition, using 'xpn' keyword instead of the 'pn', passing a 12
bytes salt using the 'salt' keyword and passing short secure channel
id (ssci) using the 'ssci' keyword as part of the ip macsec command
is required (see example).

e.g:

create a MACsec device on link eth0 with enabled xpn
  # ip link add link eth0 macsec0 type macsec port 11
	encrypt on cipher gcm-aes-xpn-128

configure a secure association on the device
  # ip macsec add macsec0 tx sa 0 xpn 1024 on ssci 5
	salt 838383838383838383838383
	key 01 81818181818181818181818181818181

configure a secure association on the device with ssci = 5
  # ip macsec add macsec0 tx sa 0 xpn 1024 on ssci 5
	salt 838383838383838383838383
	key 01 82828282828282828282828282828282

Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David Ahern <dsahern@kernel.org>