libc __sfvwrite(): roll back FILE buffer pointer on fflush error

__sfvwrite() advances the pointer before calling fflush. If fflush()
fails, it is not enough to roll back inside it, because we cannot know
how much was advanced by the caller.

Reported by: Peter <pmc@citylink.dinoex.sub.org>
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Fixes: 86a16ada1ea608408cec370171d9f59353e97c77

(cherry picked from commit bafaa70b6f9098d83d074968c8e6747ecec1e118)