Rework jumbo frame handling. QAC confirmed that the controller
requires 8 bytes alignment on RX buffer. Given that non-jumbo
frame works on any alignments I guess this DMA limitation for RX
buffer could be jumbo frame specific one. Also I'm not sure
whether this DMA limitation is related with 64bit DMA. Previously
age(4) disabled 64bit DMA addressing due to silent data corruption.
So we may need more testing on re-enabling 64bit DMA in future.

While I'm here, change mbuf chaining algorithm to use fixed sized
buffer and force software checksum if controller reports length
error. According to QAC, RFD is not updated at all for jumbo frame
so it works just like alc(4) controllers. This change also added
alignment fixup for strict alignment architectures. Because I'm
not aware of any non-x86 machines that use age(4) controllers it's
just for completeness at this moment.

Wit this change, jumbo frame should work with age(4).

Tested by: Christian Gusenbauer < c47g <> gmx dot at >
MFC after: 1 week