f2fs: add a sysfs entry to control max_discards

If frequent small discards are issued to the device, the performance would
be degraded significantly.
So, this patch adds a sysfs entry to control the number of discards to be
issued during a checkpoint procedure.

By default, f2fs does not issue any small discards, which means max_discards
is zero.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/super.c

@@ -175,6 +175,7 @@ F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_max_sleep_time, max_sleep_time);
 F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_no_gc_sleep_time, no_gc_sleep_time);
 F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_idle, gc_idle);
 F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
+F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, max_small_discards, max_discards);
 
 #define ATTR_LIST(name) (&f2fs_attr_##name.attr)
 static struct attribute *f2fs_attrs[] = {
@@ -183,6 +184,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(gc_no_gc_sleep_time),
 	ATTR_LIST(gc_idle),
 	ATTR_LIST(reclaim_segments),
+	ATTR_LIST(max_small_discards),
 	NULL,
 };