[BLOCK] blk_rq_map_sg: force clear termination bit

Since blk_rq_map_sg() sets the termination bit at the end of the sg
table, we could see it prematurely on the next mapping unless we
force drivers to do a full sg_init_table() prior to each mapping. So
force clear the termination bit to avoid having to put that clear in
the driver for every mapping.

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