Add a new encoding routine, udsl_write_cell, which extracts one ATM cell from a skb
and writes it, doing the AAL5/ATM encoding on the fly.  To make this possible, various
bits of info about the AAL5/ATM structure need to be stored beforehand in the skb.
This is done by udsl_groom_skb using the udsl_control structure introduced in the
previous patch.