librbd: get rid of AIO_STATE_CALLBACK in AioCompletion

librbd: get rid of AIO_STATE_CALLBACK in AioCompletion

After commit 002afa0fe375 ("librbd: avoid using lock within AIO
completion where possible"), the only method whose behavior would
change if AIO_STATE_CALLBACK is removed is is_complete() and it
actually needs fixing anyway: because of state != AIO_STATE_PENDING
test, is_complete() returns true both for AIO_STATE_CALLBACK and
AIO_STATE_COMPLETE, while wait_for_complete() still blocks on
AIO_STATE_CALLBACK and returns only on AIO_STATE_COMPLETE.  These
methods back public APIs, so this inconsistency is exposed to users.

If we move to setting state to AIO_STATE_COMPLETE at the top of
mark_complete_and_notify() (i.e. before event socket notification), the
transient state for callbacks can be eliminated entirely and the
inconsistency goes away.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>