io_uring: don't mark S_ISBLK async work as unbounded

S_ISBLK is marked as unbounded work for async preparation, because it
doesn't match S_ISREG. That is incorrect, as any read/write to a block
device is also a bounded operation. Fix it up and ensure that S_ISBLK
isn't marked unbounded.

Signed-off-by: Jens Axboe <axboe@kernel.dk>