sparc64: Adjust vmalloc region size based upon available virtual address bits.

In order to accomodate embedded per-cpu allocation with large numbers
of cpus and numa nodes, we have to use as much virtual address space
as possible for the vmalloc region.  Otherwise we can get things like:

PERCPU: max_distance=0x380001c10000 too large for vmalloc space 0xff00000000

So, once we select a value for PAGE_OFFSET, derive the size of the
vmalloc region based upon that.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Bob Picco <bob.picco@oracle.com>