sockbuf: Add KMSAN checks to sbappend*()

Otherwise KMSAN only detects uninitialized memory when the contents of
the buffer are copied out to userspace or transmitted to a network
interface. At that point the KMSAN violation will be far removed from
its origin, so let's try to make debugging such problems a bit easier.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38101