f2fs: enable atgc dynamically if conditions are met

Now atgc can only be enabled when umounted->mounted device
even related conditions have reached. If the device has not
be umounted->mounted for a long time, atgc can not work.

So enable atgc dynamically when atgc_age_threshold is less than
elapsed_time and ATGC mount option is on.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>