sparc64: Fix definition of VMEMMAP_SIZE.

This was the cause of various boot failures on V480, V880, etc.
systems.

Kernel image memory was being overwritten because the vmemmap[]
array was being sized to small.  So if you had physical memory
addresses past a certain point, the early bootup would spam
all over variables in the kernel data section.

The vmemmap mappings map page structs, not page struct pointers.
And that was the key thinko in the macro definition.

This was fixable thanks to the help, reports, and tireless patience
of Hermann Lauer.

Reported-by: Hermann Lauer <Hermann.Lauer@iwr.uni-heidelberg.de>
Signed-off-by: David S. Miller <davem@davemloft.net>