Correct jumbo frame support for 8168C/CP/D.

Correct jumbo frame support for 8168C/CP/D.

These newer chips use ancient design, which does _not_ support gathering RX.
An even worse aspect of the new chips' design is that it does not compat
with old ones: the buffer length field in the RX descriptor seems to be
completely ignored by the hardware.  This means host memory will be trashed
by hardware if driver uses gathering RX.

Allocate a jumbo buffer pool for these chips and configure "max RX packet
size" register according to MTU.