sctp: keep fragmentation point aligned to word size

If the user supply a different fragmentation point or if there is a
network header that cause it to not be aligned, force it to be aligned.

Fragmentation point at a value that is not aligned is not optimal.  It
causes extra padding to be used and has just no pros.

v2:
 - Make use of the new WORD_TRUNC macro
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -431,7 +431,7 @@ static inline int sctp_frag_point(const struct sctp_association *asoc, int pmtu)
 	if (asoc->user_frag)
 		frag = min_t(int, frag, asoc->user_frag);
 
-	frag = min_t(int, frag, SCTP_MAX_CHUNK_LEN);
+	frag = WORD_TRUNC(min_t(int, frag, SCTP_MAX_CHUNK_LEN));
 
 	return frag;
 }