fs: add kiocb alloc cache flag

If this kiocb can safely use the polled bio allocation cache, then this
flag must be set. Generally this can be set for polled IO, where we will
not see IRQ completions of the request.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>