brd: split I/O at page boundaries

A lot of complexity in brd stems from the fact that it tries to handle
I/O spanning two backing pages.  Instead limit the size of a single
bvec iteration so that it never crosses a page boundary and remove all
the now unneeded code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250428141014.2360063-5-hch@lst.de


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