sctp: define dst_pending_confirm as a bit in sctp_transport

As tp->dst_pending_confirm's value can only be set 0 or 1, this
patch is to change to define it as a bit instead of __u32.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>