nvme: allow user passthrough commands to poll

nvme: allow user passthrough commands to poll

The block layer knows how to deal with polled requests. Let the NVMe
driver use the previously reserved user "flags" fields to define an
option to allocate the request from the polled hardware contexts. If
polling is not enabled, then the block layer will automatically fallback
to a non-polled request.[1]

[1] https://lore.kernel.org/linux-block/20210517171443.GB2709391@dhcp-10-100-145-180.wdc.com/

Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20211220141734.12206-13-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>