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>