fflush: correct buffer handling in __sflush

Two additional stdio changes followed 86a16ada1ea6 and need to be
reverted as part of the fflush fix.

This reverts commit 6e13794fbe6e82c21365d0fd66769bf8b19c0197.
This reverts commit bafaa70b6f9098d83d074968c8e6747ecec1e118.

Fixes: d09a3bf72c0b ("fflush: correct buffer handling in __sflush")
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42491

(cherry picked from commit 4e0e01bf6511c28212d7dff94fe131a502e13026)
(cherry picked from commit d2c65a1c948648f11342274029a3f18b90aa58d2)

Approved by: so