of the packet buffer.  Although pkbuffs are guaranteed to be at least
PKB_ZLEN bytes long, there's no guarantee that the data starts at the
start of the packet buffer.

Since we have to align data for rtl8139 anyway, and since the start of
the packet buffer is guaranteed to be aligned to PKB_ALIGN, we may as
well just shuffle the packet down so that it starts at the start of
the packet buffer.  This simultaneously aligns the packet and ensures
that there is enough room for the zero-padding.