f2fs: use blk_plug in all the possible paths

This patch reverts 19a5f5e2 (f2fs: drop any block plugging),
and adds blk_plug in write paths additionally.

The main reason is that blk_start_plug can be used to wake up from low-power
mode before submitting further bios.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>