sctp: convert sack_needed and sack_generation to bits

They don't need to be any bigger than that and with this we start a new
bitfield for tracking association runtime stuff, like zero window
situation.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>