sctp: make use of SCTP_TRUNC4 macro

And avoid the usage of '&~3'. This is the last place still not using
the macro.
Also break the line to make it easier to read.

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