page_to_virt and lowmem_page_address provided equiavlent functionality
so use the more standard lowmem_page_address

This also addresses build issue in ARCH=powerpc since page_to_virt()
has been removed from include/asm-powerpc/page.h

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>