tcp: remove useless add operation when init sysctl_max_tw_buckets

cp_hashinfo.ehash_mask is always an odd number, which is set in function
alloc_large_system_hash(). See bellow,
        if (_hash_mask)
                *_hash_mask = (1 << log2qty) - 1; <<< always odd number

Hence the local variable 'cnt' is a even number, as a result of that it is
no difference to do the incrementation here.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>