block: add a bdev_rw_virt helper

block: add a bdev_rw_virt helper

Add a helper to perform synchronous I/O on a kernel direct map range.
Currently this is implemented in various places in usually not very
efficient ways, so provide a generic helper instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250507120451.4000627-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>