Eliminate creation/destruction of dmamap's for every xmit by reusing statically allocated maps.
Eliminate creation/destruction of dmamap's for every xmit by reusing
statically allocated maps.

Significant performance boost on both sparc64 and macppc.

Tested on macppc by drahn@.

ok jason@ drahn@.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars