Simplify macros EFPRINTF and EFFLUSH. [0]
Also stdarg(3) says that each invocation of va_start() must be paired
with a corresponding invocation of va_end() in the same function. [1]

Reported by: Coverity
CID: 1194318[0] and 1194332[1]
Discussed with: jhb
MFC after: 4 weeks.
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D15548