zram: do not forget to endio for partial discard requests

In the Linux kernel, the following vulnerability has been resolved:

zram: do not forget to endio for partial discard requests

As reported by Qu Wenruo and Avinesh Kumar, the following

getconf PAGESIZE

65536

blkdiscard -p 4k /dev/zram0

takes literally forever to complete. zram doesn't support partial

discards and just returns immediately w/o doing any discard work in such

cases. The problem is that we forget to endio on our way out, so

blkdiscard sleeps forever in submit_bio_wait(). Fix this by jumping to

end_bio label, which does bio_endio().