io_uring: enable use of bio alloc cache

Mark polled IO as being safe for dipping into the bio allocation
cache, in case the targeted bio_set has it enabled.

This brings an IOPOLL gen2 Optane QD=128 workload from ~3.2M IOPS to
~3.5M IOPS.

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