mm: simplify swapdev_block

Open code the parts of map_swap_entry that was actually used by
swapdev_block, and remove the now unused map_swap_entry function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>