sparc64: Use kernel page tables for vmemmap.

sparc64: Use kernel page tables for vmemmap.

[ Upstream commit c06240c7f5c39c83dfd7849c0770775562441b96 ]

For sparse memory configurations, the vmemmap array behaves terribly
and it takes up an inordinate amount of space in the BSS section of
the kernel image unconditionally.

Just build huge PMDs and look them up just like we do for TLB misses
in the vmalloc area.

Kernel BSS shrinks by about 2MB.

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