powerpc/mm/book3s64: Fix build error with SPARSEMEM disabled

With CONFIG_SPARSEMEM disabled the below kernel build error is observed.

 arch/powerpc/mm/init_64.c:477:38: error: use of undeclared identifier 'SECTION_SIZE_BITS'

CONFIG_MEMORY_HOTPLUG depends on CONFIG_SPARSEMEM and it is more clear
to describe the code dependency in terms of MEMORY_HOTPLUG. Outside
memory hotplug the kernel uses memory_block_size for kernel directmap.
Instead of depending on SECTION_SIZE_BITS to compute the direct map
page size, add a new #define which defaults to 16M(same as existing
SECTION_SIZE)

Fixes: 4d15721 ("powerpc/mm: Cleanup memory block size probing")
Signed-off-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308251532.k9PpWEAD-lkp@intel.com/
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230828074658.59553-1-aneesh.kumar@linux.ibm.com

Author: kvaneesh

arch/powerpc/mm/init_64.c

@@ -472,12 +472,23 @@ static int __init dt_scan_mmu_pid_width(unsigned long node,
 	return 1;
 }
 
+/*
+ * Outside hotplug the kernel uses this value to map the kernel direct map
+ * with radix. To be compatible with older kernels, let's keep this value
+ * as 16M which is also SECTION_SIZE with SPARSEMEM. We can ideally map
+ * things with 1GB size in the case where we don't support hotplug.
+ */
+#ifndef CONFIG_MEMORY_HOTPLUG
+#define DEFAULT_MEMORY_BLOCK_SIZE	SZ_16M
+#else
+#define DEFAULT_MEMORY_BLOCK_SIZE	MIN_MEMORY_BLOCK_SIZE
+#endif
+
 static void update_memory_block_size(unsigned long *block_size, unsigned long mem_size)
 {
-	unsigned long section_size = 1UL << SECTION_SIZE_BITS;
-
-	for (; *block_size > section_size; *block_size >>= 2) {
+	unsigned long min_memory_block_size = DEFAULT_MEMORY_BLOCK_SIZE;
 
+	for (; *block_size > min_memory_block_size; *block_size >>= 2) {
 		if ((mem_size & *block_size) == 0)
 			break;
 	}
@@ -507,7 +518,7 @@ static int __init probe_memory_block_size(unsigned long node, const char *uname,
 			/*
 			 * Nothing in the device tree
 			 */
-			*block_size = MIN_MEMORY_BLOCK_SIZE;
+			*block_size = DEFAULT_MEMORY_BLOCK_SIZE;
 		else
 			*block_size = of_read_number(prop, dt_root_size_cells);
 		/*