Revert "Rewrite and simplify iscsi_iovector_readv_writev"

Since writing headers and payload in a single iov has never been
implementend and after thinking about it several times seems to
be very hairy I would like to revert this change since
the original implementation is in O(1) while the changed one
is in O(n). This results in a complexity of O(n^2) instead of
O(n) for the whole send operation.

This reverts commit 06eab264f64fccec76553c136db8608e8c069495.