powerpc/64s: Fix boot failure with 4K Radix

When using the Radix MMU our PGD is always 64K, and must be naturally
aligned.

For a 4K page size kernel that means page alignment of swapper_pg_dir is
not sufficient, leading to failure to boot.

Use the existing MAX_PTRS_PER_PGD which has the correct value, and
avoids us hard-coding 64K here.

Fixes: e72421a0 ("powerpc: Define swapper_pg_dir[] in C")
Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210624123420.2784187-1-mpe@ellerman.id.au

arch/powerpc/mm/pgtable.c

@@ -28,7 +28,13 @@
 #include <asm/hugetlb.h>
 #include <asm/pte-walk.h>
 
-pgd_t swapper_pg_dir[MAX_PTRS_PER_PGD] __page_aligned_bss;
+#ifdef CONFIG_PPC64
+#define PGD_ALIGN (sizeof(pgd_t) * MAX_PTRS_PER_PGD)
+#else
+#define PGD_ALIGN PAGE_SIZE
+#endif
+
+pgd_t swapper_pg_dir[MAX_PTRS_PER_PGD] __section(".bss..page_aligned") __aligned(PGD_ALIGN);
 
 static inline int is_exec_fault(void)
 {