mka: Clean up key allocation

mka: Clean up key allocation

Assign cs in ieee802_1x_mka_decode_dist_sak_body and reuse it.

Cleanup of key allocation: ieee802_1x_kay_generate_new_sak() and
ieee802_1x_mka_decode_dist_sak_body() both allocate a struct key_conf,
fill it, and ask ieee802_1x_kay_init_data_key() to allocate and set up a
struct data_key. They also allocate multiple key buffers and copy the
same data around. Stop moving data from buffer to buffer, and just
allocate what we really need.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>