net: macsec: Expose MACSEC_SALT_LEN definition to user space

Expose MACSEC_SALT_LEN definition to user space to be
used in various user space applications such as iproute.
Iproute will use this as part of adding macsec extended
packet number support.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Link: https://lore.kernel.org/r/20220818153229.4721-1-ehakim@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>