blk-mq: place trace_block_getrq() in correct place

trace_block_getrq() is to indicate a request struct has been allocated
for queue, so put it in right place.

Reviewed-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>