s390/memory hotplug: provide memory_block_size_bytes() function

Commit 0c2c99b1 "memory hotplug: Allow memory blocks to span
multiple memory sections" introduced a weak memory_block_size_bytes()
function which can be used to set the size of a memory block as
seen in sysfs.
Provide an s390 specific override which makes sure that each
memory block has at least a size of 256MB or the increment size of
of a memory increment, whatever is larger.
This way we can make sure that the number of memory sysfs objects
doesn't explode for very large memory configurations.
Reported-by: NGerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/init.c

@@ -21,6 +21,7 @@
 #include <linux/init.h>
 #include <linux/pagemap.h>
 #include <linux/bootmem.h>
+#include <linux/memory.h>
 #include <linux/pfn.h>
 #include <linux/poison.h>
 #include <linux/initrd.h>
@@ -36,6 +37,7 @@
 #include <asm/tlbflush.h>
 #include <asm/sections.h>
 #include <asm/ctl_reg.h>
+#include <asm/sclp.h>
 
 pgd_t swapper_pg_dir[PTRS_PER_PGD] __attribute__((__aligned__(PAGE_SIZE)));
 
@@ -214,6 +216,15 @@ int arch_add_memory(int nid, u64 start, u64 size)
 	return rc;
 }
 
+unsigned long memory_block_size_bytes(void)
+{
+	/*
+	 * Make sure the memory block size is always greater
+	 * or equal than the memory increment size.
+	 */
+	return max_t(unsigned long, MIN_MEMORY_BLOCK_SIZE, sclp_get_rzm());
+}
+
 #ifdef CONFIG_MEMORY_HOTREMOVE
 int arch_remove_memory(u64 start, u64 size)
 {