previously just always write over the beginning of the buffer for each vector.
Since the writev version isn't exposed to userland by means of a syscall and
kernel internally nobody used it, nobody noticed so far.
* Merge the two loops for user and kernel copy to remove the code duplication.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@42273 a95241bf-73f2-0310-859d-f6bbb57e9c96