aio: correct calculation of available events

aio: correct calculation of available events

When the number of available events in the ring buffer is calculated, the
avail calculation is incorrect when head == tail.  This is harmless in
aio_read_events_ring(), but in free_ioctx() leads to the subsequent
WARN_ON(atomic_read(&ctx->reqs_available) > ctx->nr).  Correct this.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>