In case of buffered reading from block device, when short read happens,
we should retry to read more, otherwise the IO will be completed
partially, for example, the following fio expects to read 2MB, but it
can only read 1M or less bytes:

    fio --name=onessd --filename=/dev/nvme0n1 --filesize=2M \
	--rw=randread --bs=2M --direct=0 --overwrite=0 --numjobs=1 \
	--iodepth=1 --time_based=0 --runtime=2 --ioengine=io_uring \
	--registerfiles --fixedbufs --gtod_reduce=1 --group_reporting

Fix the issue by allowing short read retry for block device, which sets
FMODE_BUF_RASYNC really.

Fixes: 9a173346 ("io_uring: fix short read retries for non-reg files")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Reviewed-by: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210821150751.1290434-1-ming.lei@redhat.comSigned-off-by: NJens Axboe <axboe@kernel.dk>