nvmet: add plugging for read/write when ns is bdev

With reference to the following issue reported on the mailing list :-
http://lists.infradead.org/pipermail/linux-nvme/2019-October/027604.html
this patch adds plugging for the bdev-ns under nvmet_bdev_execute_rw().

We can see the following performance improvement in random write
workload I/Os with the setup described in the link when device_path
configured as /dev/md0.

Without this patch :-

  write: IOPS=40.8k, BW=159MiB/s (167MB/s)(4777MiB/30002msec)
  write: IOPS=41.2k, BW=161MiB/s (169MB/s)(4831MiB/30011msec)
    slat (usec): min=8,  max=10823, avg=15.64,  stdev=16.85
    slat (usec): min=8,  max=401,   avg=15.40,  stdev= 9.56
    clat (usec): min=54, max=2492,  avg=759.07, stdev=172.62
    clat (usec): min=56, max=1997,  avg=768.06, stdev=178.72

With this patch :-

  write: IOPS=123k, BW=480MiB/s (504MB/s)(14.1GiB/30011msec)
  write: IOPS=123k, BW=481MiB/s (504MB/s)(14.1GiB/30002msec)
    slat (usec): min=8,  max=9941,  avg=13.31,  stdev= 8.04
    slat (usec): min=8,  max=289,   avg=13.31,  stdev= 3.37
    clat (usec): min=43, max=17635, avg=245.46, stdev=171.23
    clat (usec): min=44, max=17751, avg=245.25, stdev=183.14

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/io-cmd-bdev.c

@@ -147,6 +147,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
 	int sg_cnt = req->sg_cnt;
 	struct bio *bio;
 	struct scatterlist *sg;
+	struct blk_plug plug;
 	sector_t sector;
 	int op, op_flags = 0, i;
 
@@ -185,6 +186,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
 	bio->bi_end_io = nvmet_bio_done;
 	bio_set_op_attrs(bio, op, op_flags);
 
+	blk_start_plug(&plug);
 	for_each_sg(req->sg, sg, req->sg_cnt, i) {
 		while (bio_add_page(bio, sg_page(sg), sg->length, sg->offset)
 				!= sg->length) {
@@ -204,6 +206,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
 	}
 
 	submit_bio(bio);
+	blk_finish_plug(&plug);
 }
 
 static void nvmet_bdev_execute_flush(struct nvmet_req *req)