f2fs: much larger batched trim_fs job

We have a kernel thread to issue discard commands, so we can increase the
number of batched discard sections. By default, now it becomes 4GB range.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -128,7 +128,7 @@ enum {
 	CP_DISCARD,
 };
 
-#define DEF_BATCHED_TRIM_SECTIONS	2
+#define DEF_BATCHED_TRIM_SECTIONS	2048
 #define BATCHED_TRIM_SEGMENTS(sbi)	\
 		(SM_I(sbi)->trim_sections * (sbi)->segs_per_sec)
 #define BATCHED_TRIM_BLOCKS(sbi)	\