The condition of the for loop is in the middle – between the two semicolons ;.
In C++ it is OK to put almost any expression as a condition: anything that evaluates to zero means false; non-zero means true.
In your case, the condition is u--: when you convert to C#, simply add != 0:
for (u = b.size(), v = b.back(); u-- != 0; v = p[v])
b[u] = v; // ^^^^ HERE