Fix totally broken put_vformat() implementation

I'm ashamed.  put_vbuffer() worked perfectly fine for the normal case
when everything fit in our simple buffer on-stack, but the fallback case
was broken in so many ways that I'm just going to go sit in a corner and
cry myself to sleep.

And dammit, I _knew_ how to do it right.  I knew you had to do a
"va_copy()" and couldn't just keep re-using 'args'.  I've done this
before.  But I half-arsed it, and nobody ever noticed, because you
wouldn't do C style format strings for big strings.

"128 bytes is enough for everybody".

And as penance for this idiocy, I just spent too much time trying to
figure out what was wrong in my git loading code when my debug printouts
caused SIGSEGV's.

Sigh.

Anyway, now it should hopefully be correct, and the code is smarter
about things too, not having that extra buffer since we already *have* a
buffer in the "struct membuffer" we are working with.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>