block: don't plug in blkdev_write_iter

For direct I/O writes that issues more than a single bio, the plugging
is already done in __blkdev_direct_IO.
For synchronous buffered writes the plugging is done deep down in
writeback_inodes_wb / wb_writeback.

For the other cases there is no point in plugging as as single bio or no
bio at all is submitted.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230520044503.334444-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -520,7 +520,6 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct block_device *bdev = iocb->ki_filp->private_data;
 	struct inode *bd_inode = bdev->bd_inode;
 	loff_t size = bdev_nr_bytes(bdev);
-	struct blk_plug plug;
 	size_t shorted = 0;
 	ssize_t ret;
 
@@ -545,12 +544,10 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		iov_iter_truncate(from, size);
 	}
 
-	blk_start_plug(&plug);
 	ret = __generic_file_write_iter(iocb, from);
 	if (ret > 0)
 		ret = generic_write_sync(iocb, ret);
 	iov_iter_reexpand(from, iov_iter_count(from) + shorted);
-	blk_finish_plug(&plug);
 	return ret;
 }