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: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>