[POWERPC] Size swapper_pg_dir correctly

David Gibson pointed out that swapper_pg_dir actually need to be
PGD_TABLE_SIZE bytes long not PAGE_SIZE.  This actually saves 64k in
the bss for a kernel ppc64_defconfig built with CONFIG_PPC_64K_PAGES.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>