sctp: refactor sctp_datamsg_from_user

This patch refactors sctp_datamsg_from_user() in an attempt to make it
better to read and avoid code duplication for handling the last
fragment.

It also avoids doing division and remaining operations. Even though, it
should still operate similarly as before this patch.

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