sparc32: drop swapper_pg_dir

We save one page of RAM dropping swapper_pg_dir.
It was only used for an assignment in init-mm.c and we
redid this later in srmmu.c anyway.

This is likely a left-over from the sun4c removal.

To avoid a dummy variable we use a simple #define swapper_pg_dir NULL

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>