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>

include/trace/events/f2fs.h

@@ -72,7 +72,8 @@
 #define show_cpreason(type)						\
 	__print_symbolic(type,						\
 		{ CP_UMOUNT,	"Umount" },				\
-		{ CP_SYNC,	"Sync" })
+		{ CP_SYNC,	"Sync" },				\
+		{ CP_DISCARD,	"Discard" })
 
 struct victim_sel_policy;