f2fs: optimize __update_nat_bits

Make three modification for __update_nat_bits:
1. Take the codes of dealing the nat with nid 0 out of the loop
    Such nat only needs to be dealt with once at beginning.
2. Use " nat_index == 0" instead of " start_nid == 0" to decide if it's the first nat block
    It's better that we don't assume @start_nid is the first nid of the nat block it's in.
3. Use " if (nat_blk->entries[i].block_addr != NULL_ADDR)" to explicitly comfirm the value of block_addr
    use constant to make sure the codes is right, even if the value of NULL_ADDR changes.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -2421,15 +2421,17 @@ static void __update_nat_bits(struct f2fs_sb_info *sbi, nid_t start_nid,
 	unsigned int nat_index = start_nid / NAT_ENTRY_PER_BLOCK;
 	struct f2fs_nat_block *nat_blk = page_address(page);
 	int valid = 0;
-	int i;
+	int i = 0;
 
 	if (!enabled_nat_bits(sbi, NULL))
 		return;
 
-	for (i = 0; i < NAT_ENTRY_PER_BLOCK; i++) {
-		if (start_nid == 0 && i == 0)
-			valid++;
-		if (nat_blk->entries[i].block_addr)
+	if (nat_index == 0) {
+		valid = 1;
+		i = 1;
+	}
+	for (; i < NAT_ENTRY_PER_BLOCK; i++) {
+		if (nat_blk->entries[i].block_addr != NULL_ADDR)
 			valid++;
 	}
 	if (valid == 0) {