MFC r223930:

Remove include of sys/sbuf.h from geom/geom.h.
sbuf support is not always required for geom/geom.h users, and no need to
depend from it.

PR:		158398
Revert r268415.

I botched testing and a large set of changes needs to be applied for this to
work.
Fix kernel memory disclosure in control message and SCTP notifications.

Security:	FreeBSD-SA-14:17.kmem
Security:	CVE-2014-3952, CVE-2014-3953
Bump __FreeBSD_version after SA-14:17.kmem so we have something
to test against in the freebsd.org cluster.