vmalloc and pkmap overlap on 2.4

[Peng Dai]

The vmalloc pool runs from 0xf8000000 to FIXADDR_START,
which is a few pages below the top of virtual address space.

The pkmap area is from 0xfe000000 to 0xfe400000, which fits
within the vmalloc pool.

There does not seem to be any mechanism to prevent these two
from stepping on each other's toes. It could be fixed by, for
example, creating a  vm_struct that covers the pkmap area and
enlisting it to vmlist.

Thanks
Peng