block_write_full_page: switch synchronous writes to use WRITE_SYNC_PLUG

 * locked buffer.   This only can happen if someone has written the buffer

 * directly, with submit_bh().  At the address_space level PageWriteback

 * prevents this contention from occurring.

 *

 * If block_write_full_page() is called with wbc->sync_mode ==

 * WB_SYNC_ALL, the writes are posted using WRITE_SYNC_PLUG; this

 * causes the writes to be flagged as synchronous writes, but the

 * block device queue will NOT be unplugged, since usually many pages

 * will be pushed to the out before the higher-level caller actually

 * waits for the writes to be completed.  The various wait functions,

 * such as wait_on_writeback_range() will ultimately call sync_page()

 * which will ultimately call blk_run_backing_dev(), which will end up

 * unplugging the device queue.

 */

static int __block_write_full_page(struct inode *inode, struct page *page,

			get_block_t *get_block, struct writeback_control *wbc)

	struct buffer_head *bh, *head;

	const unsigned blocksize = 1 << inode->i_blkbits;

	int nr_underway = 0;

	int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);

	int write_op = (wbc->sync_mode == WB_SYNC_ALL ?

			WRITE_SYNC_PLUG : WRITE);

	BUG_ON(!PageLocked(page));