f2fs: trival cleanup in move_data_block()

Trival cleanups:
- relocate set_summary() before its use
- relocate "allocate block address" to correct place
- remove unneeded f2fs_wait_on_page_writeback()

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