f2fs: submit bio at the reclaim path

If f2fs_write_data_page is called through the reclaim path, we should submit
the bio right away.

This patch resolves the following issue that Marc Dietrich reported.
"It took me a while to bisect a problem which causes my ARM (tegra2) netbook to
frequently stall for 5-10 seconds when I enable EXA acceleration (opentegra
experimental ddx)."
And this patch fixes that.
Reported-by: NMarc Dietrich <marvin24@gmx.de>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/data.c

@@ -833,6 +833,8 @@ static int f2fs_write_data_page(struct page *page,
 	unlock_page(page);
 	if (need_balance_fs)
 		f2fs_balance_fs(sbi);
+	if (wbc->for_reclaim)
+		f2fs_submit_merged_bio(sbi, DATA, WRITE);
 	return 0;
 
 redirty_out: