sctp: rework switch cases in sctp_outq_flush_data

Remove an inner one, which tended to be error prone due to the cascading
and it can be replaced by a simple if ().

Rework the outer one so that the actual flush code is not inside it. Now
we first validate if we can or cannot send data, return if not, and then
the flush code.

Suggested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>