coroutine: Move total initialization outside coroutine

coroutine: Move total initialization outside coroutine

The sample coroutine in api-3 initializes a total to 0, then adds up the
pseudo-random data it has placed into a stack buffer, to ensure that the
compiler won't elide the reading and writing of that buffer.  After the
coroutine has completed, we verify that total is non-zero so that we'll
detect if the coroutine failed to execute entirely.

Except that the initialization of total is within the coroutine itself,
so it could also be non-zero due to it simply being uninitialized.  This
moves the initialization outside the coroutine, to make the test a little
more robust.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>