__sflush(): on write error, if nothing was written, reset FILE state back

otherwise the data is just dropped. Check for current position equal to
the buffer base at the entry of the function; if not equal, setvbuf()
was done from the write method and it is not our business to override
the decision.

PR: 76398
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34007