BUG/MEDIUM: htx: Fix headers rollback on partial copy in htx_xfer()

BUG/MEDIUM: htx: Fix headers rollback on partial copy in htx_xfer()

In htx_xfer() function, when headers are partially copied, depending on the
flags, a rollback may be performed to remove all copied headers from the
destination message. However, there was an issue in the loop performing the
rollback. Instead of decrementing the returned value using the size of the
HTX block from the destination message, the one from the source message was
used. So the wrong value was be returned and in worst case, it could
overflow.

In addition, the BUG_ON() in the loop was removed because test condition was
wrong.

It is a 3.4-specific issue. No backport needed.