zdtm/atomic: atomic_inc should return the old value

00:24:30.269:    44: ERR: test.c:208: Test exited on unexpected signal

It's a correct name for this function. For example, we use this name
here criu/arch/ppc64/include/asm/atomic.h

And you can find the same set of functions here:
test/zdtm/lib/arch/aarch64/include/asm/atomic.h

I think this code is more readable (understandable) than just
return t - 1.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>