igb: Free tx mbufs proactively.

igb: Free tx mbufs proactively.

For 82575, which is earliest product of this product line, the RS
bit is set on every packet's last TX desc by default, since in the
'head write back' mode, the content of TDH register does not move,
if the content of the 'header', which is memory based, does not
move.  It is still allowed to reduce the density of RS TX descs,
which will be useful for workloads w/o using sendfile.

This is preparation for the dillon's upcoming sendfile patch.