block: Use enum to define RQF_x bit indexes

Similar to what we do for enum req_flag_bits, divide the definition of
RQF_x flags into an enum to declare the bits and an actual flag.

Tweak some comments to not spill onto new lines.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240719112912.3830443-14-john.g.garry@oracle.com


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