MFC r333603:

Ensure that the MTU's used are multiple of 4.

The length of SCTP packets is always a multiple of 4. Therefore,
ensure that the MTUs used are also a multiple of 4.

Thanks to Irene Ruengeler for providing an earlier version of this
patch.

MFC r334286:

Use correct mask.
Introduced in https://svnweb.freebsd.org/changeset/base/333603.
Thanks to Irene Ruengler for testing and reporting the issue.