f2fs: introduce FITRIM in f2fs_ioctl

This patch introduces FITRIM in f2fs_ioctl.
In this case, f2fs will issue small discards and prefree discards as many as
possible for the given area.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/file.c
Simple merge

diff --cc fs/f2fs/segment.c
Simple merge

diff --cc fs/f2fs/super.c
Simple merge

diff --cc include/trace/events/f2fs.h
Simple merge