aio: all callers of aio_{read,write,fsync,poll} treat 0 and -EIOCBQUEUED the same way

... so just make them return 0 when caller does not need to destroy iocb

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>