aio: fix ringbuffer calculation so we don't wrap

aio: fix ringbuffer calculation so we don't wrap

When calculating the size of the ringbuffer, aio_setup_ring() adds 2 to
nr_events so that the head and tail entries don't overlap when the
ringbuffer fills up.

But then later the code uses the calculated ringbuffer size for the
maximum number of outstanding kiocbs, so this buffer doesn't actually get
used.  Ouch.

Fix it so the ringbuffer size and maximum number of requests are distinct
again, and document things while we're at it.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Zach Brown <zab@redhat.com>
Cc: Josh Boyer <jwboyer@redhat.com>
Cc: Zach Brown <zab@redhat.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>