null_blk: Have all null_handle_xxx() return a blk_status_t

Modify the null_handle_flush() and null_handle_rq() functions to return
a blk_status_t instead of an errno to simplify the call sites of these
functions and to be consistant with other null_handle_xxx() functions.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Nitesh Shetty <nj.shetty@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20240411085502.728558-2-dlemoal@kernel.org


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