f2fs: fix to avoid redundant discard during fstrim

With below test steps, f2fs will issue redundant discard when doing fstrim,
the reason is that we issue discards for both prefree segments and
consecutive freed region user wants to trim, part regions they covered are
overlapped, here, we change to do not to issue any discards for prefree
segments in trimmed range.

1. mount -t f2fs -o discard /dev/zram0 /mnt/f2fs
2. fstrim -o 0 -l 3221225472 -m 2097152 -v /mnt/f2fs/
3. dd if=/dev/zero  of=/mnt/f2fs/a bs=2M count=1
4. dd if=/dev/zero  of=/mnt/f2fs/b bs=1M count=1
5. sync
6. rm /mnt/f2fs/a /mnt/f2fs/b
7. fstrim -o 0 -l 3221225472 -m 2097152 -v /mnt/f2fs/

Before:
<...>-5428  [001] ...1  9511.052125: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x200
<...>-5428  [001] ...1  9511.052787: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x300

After:
<...>-6764  [000] ...1  9720.382504: f2fs_issue_discard: dev = (251,0), blkstart = 0x2200, blklen = 0x300
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -714,6 +714,7 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	unsigned long *prefree_map = dirty_i->dirty_segmap[PRE];
 	unsigned int start = 0, end = -1;
 	unsigned int secno, start_segno;
+	bool force = (cpc->reason == CP_DISCARD);
 
 	mutex_lock(&dirty_i->seglist_lock);
 
@@ -730,7 +731,7 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 
 		dirty_i->nr_dirty[PRE] -= end - start;
 
-		if (!test_opt(sbi, DISCARD))
+		if (force || !test_opt(sbi, DISCARD))
 			continue;
 
 		if (!test_opt(sbi, LFS) || sbi->segs_per_sec == 1) {
@@ -754,7 +755,7 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 
 	/* send small discards */
 	list_for_each_entry_safe(entry, this, head, list) {
-		if (cpc->reason == CP_DISCARD && entry->len < cpc->trim_minlen)
+		if (force && entry->len < cpc->trim_minlen)
 			goto skip;
 		f2fs_issue_discard(sbi, entry->blkaddr, entry->len);
 		cpc->trimmed += entry->len;