f2fs: record average update time of segment

Previously, once we update one block in segment, we will update mtime of
segment to last time, making aged segment becoming freshest, result in
that GC with cost benefit algorithm missing such segment, So this patch
changes to record mtime as average block updating time instead of last
updating time.

It's not needed to reset mtime for prefree segment, as se->valid_blocks
is zero, then old se->mtime won't take any weight with below calculation:

	se->mtime = div_u64(se->mtime * se->valid_blocks + mtime,
					se->valid_blocks + 1);

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