powerpc/mm: Move CMA reservations after initmem_init()

After commit 11ac3e87 ("mm: cma: use pageblock_order as the single
alignment") there is an error at boot about the KVM CMA reservation
failing, eg:

  kvm_cma_reserve: reserving 6553 MiB for global area
  cma: Failed to reserve 6553 MiB

That makes it impossible to start KVM guests using the hash MMU with
more than 2G of memory, because the VM is unable to allocate a large
enough region for the hash page table, eg:

  $ qemu-system-ppc64 -enable-kvm -M pseries -m 4G ...
  qemu-system-ppc64: Failed to allocate KVM HPT of order 25: Cannot allocate memory

Aneesh pointed out that this happens because when kvm_cma_reserve() is
called, pageblock_order has not been initialised yet, and is still zero,
causing the checks in cma_init_reserved_mem() against
CMA_MIN_ALIGNMENT_PAGES to fail.

Fix it by moving the call to kvm_cma_reserve() after initmem_init(). The
pageblock_order is initialised in sparse_init() which is called from
initmem_init().

Also move the hugetlb CMA reservation.

Fixes: 11ac3e87 ("mm: cma: use pageblock_order as the single alignment")
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220616120033.1976732-1-mpe@ellerman.id.au

arch/powerpc/kernel/setup-common.c

@@ -935,12 +935,6 @@ void __init setup_arch(char **cmdline_p)
 	/* Print various info about the machine that has been gathered so far. */
 	print_system_info();
 
-	/* Reserve large chunks of memory for use by CMA for KVM. */
-	kvm_cma_reserve();
-
-	/*  Reserve large chunks of memory for us by CMA for hugetlb */
-	gigantic_hugetlb_cma_reserve();
-
 	klp_init_thread_info(&init_task);
 
 	setup_initial_init_mm(_stext, _etext, _edata, _end);
@@ -955,6 +949,13 @@ void __init setup_arch(char **cmdline_p)
 
 	initmem_init();
 
+	/*
+	 * Reserve large chunks of memory for use by CMA for KVM and hugetlb. These must
+	 * be called after initmem_init(), so that pageblock_order is initialised.
+	 */
+	kvm_cma_reserve();
+	gigantic_hugetlb_cma_reserve();
+
 	early_memtest(min_low_pfn << PAGE_SHIFT, max_low_pfn << PAGE_SHIFT);
 
 	if (ppc_md.setup_arch)