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.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>