f2fs: remove redundant block plug

For buffered IO, we don't need to use block plug to cache bio,
for direct IO, generic f2fs_direct_IO has already added block
plug, so let's remove redundant one in .write_iter.

As Yunlei described in his patch:

-f2fs_file_write_iter
  -blk_start_plug
    -__generic_file_write_iter
	...
	  -do_blockdev_direct_IO
	    -blk_start_plug
		...
	    -blk_finish_plug
	...
  -blk_finish_plug

which may conduct performance decrease in our platform

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>