f2fs: validate before set/clear free nat bitmap

In flush_nat_entries, all dirty nats will be flushed and if
their new address isn't NULL_ADDR, their bitmaps will be updated,
the free_nid_count of the bitmaps will be increaced regardless
of whether the nats have already been occupied before.
This could lead to wrong free_nid_count.
So this patch checks the status of the bits beforeactually
set/clear them.

Fixes: 586d1492 ("f2fs: skip scanning free nid bitmap of full NAT blocks")
Signed-off-by: NFan li <fanofcode.li@samsung.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1906,15 +1906,18 @@ static void update_free_nid_bitmap(struct f2fs_sb_info *sbi, nid_t nid,
 	if (!test_bit_le(nat_ofs, nm_i->nat_block_bitmap))
 		return;
 
-	if (set)
+	if (set) {
+		if (test_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]))
+			return;
 		__set_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]);
-	else
-		__clear_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]);
-
-	if (set)
 		nm_i->free_nid_count[nat_ofs]++;
-	else if (!build)
-		nm_i->free_nid_count[nat_ofs]--;
+	} else {
+		if (!test_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]))
+			return;
+		__clear_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]);
+		if (!build)
+			nm_i->free_nid_count[nat_ofs]--;
+	}
 }
 
 static void scan_nat_page(struct f2fs_sb_info *sbi,