f2fs: should update REQ_TIME for direct write

The sending interval of discard and GC should also
consider direct write requests; filesystem is not
idle if there is direct write.

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