Use m_collapse(9) instead of m_defrag(9) in e1000 drivers

[marius]

Summary

Given that em(4), lem(4) and igb(4) hardware doesn't require the
alignment guarantees provided by m_defrag(9), use m_collapse(9)
instead for performance reasons.
While at it, sanitize the statistics softc members, i. e. retire
unused ones and add SYSCTL nodes missing for actually used ones.

[sbruno]

This looks fine and an improvement.