blk-mq: fix FUA request hang

When a FUA request enters its DATA stage of flush pipeline, the
request is added to mq requeue list, the request will then be added to
ctx->rq_list. blk_mq_attempt_merge() might merge the request with a bio.
Later when the request is finished the flush pipeline, the
request->__data_len is 0. Then I only saw the bio gets endio called, the
original request never finish.

Adding REQ_FLUSH_SEQ into REQ_NOMERGE_FLAGS looks an easy fix.

stable: 3.15+

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