sd/skd: stuff discard page in request->completion_data

Store the pointer to the page there, so we can always safely
reference it from end_io context where ->bio may have been
cleared.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc drivers/block/skd_main.c
Simple merge

diff --cc drivers/scsi/sd.c
Simple merge