sparc64: Define WANT_PAGE_VIRTUAL

sparc64: Define WANT_PAGE_VIRTUAL

As sparse warns, without this struct page pointer subtraction is
extremely expensive, and this is a pretty common operation in
fast paths.

With this define struct page becomes 64 bytes which makes for a
simple subtract and shift, instead of a costly divide or reciprocol
multiply.

Signed-off-by: David S. Miller <davem@davemloft.net>