macsec: Support XPN frame handling - IEEE 802.1AEbw

Support extended packet number cipher suites (802.1AEbw) frames handling.
This does not include the needed netlink patches.

    * Added xpn boolean field to `struct macsec_secy`.
    * Added ssci field to `struct_macsec_tx_sa` (802.1AE figure 10-5).
    * Added ssci field to `struct_macsec_rx_sa` (802.1AE figure 10-5).
    * Added salt field to `struct macsec_key` (802.1AE 10.7 NOTE 1).
    * Created pn_t type for easy access to lower and upper halves.
    * Created salt_t type for easy access to the "ssci" and "pn" parts.
    * Created `macsec_fill_iv_xpn` function to create IV in XPN mode.
    * Support in PN recovery and preliminary replay check in XPN mode.

In addition, according to IEEE 802.1AEbw figure 10-5, the PN of incoming
frame can be 0 when XPN cipher suite is used, so fixed the function
`macsec_validate_skb` to fail on PN=0 only if XPN is off.

Signed-off-by: Era Mayflower <mayflowerera@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>