f2fs: do not discard data protected by the previous checkpoint

We should not discard any data protected by the previous checkpoint all
the time.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -517,7 +517,7 @@ static void add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 
 	/* SIT_VBLOCK_MAP_SIZE should be multiple of sizeof(unsigned long) */
 	for (i = 0; i < entries; i++)
-		dmap[i] = (cur_map[i] ^ ckpt_map[i]) & ckpt_map[i];
+		dmap[i] = ~(cur_map[i] | ckpt_map[i]);
 
 	while (force || SM_I(sbi)->nr_discards <= SM_I(sbi)->max_discards) {
 		start = __find_rev_next_bit(dmap, max_blocks, end + 1);