s390/os_info: Fix array size in struct os_info

gcc's -Warray-bounds warned about an out-of-bounds access to
the entry array contained in struct os_info. This doesn't trigger
a bug right now because there's a large reserved space after the
array. Nevertheless fix this, and also add a BUILD_BUG_ON to make
sure struct os_info is always exactly on page in size.

Fixes: f4cac27d ("s390/crash: Use old os_info to create PT_LOAD headers")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/include/asm/os_info.h

@@ -27,6 +27,7 @@
 #define OS_INFO_IMAGE_START	10
 #define OS_INFO_IMAGE_END	11
 #define OS_INFO_IMAGE_PHYS	12
+#define OS_INFO_MAX		13
 
 #define OS_INFO_FLAG_REIPL_CLEAR	(1UL << 0)
 
@@ -46,8 +47,8 @@ struct os_info {
 	u16	version_minor;
 	u64	crashkernel_addr;
 	u64	crashkernel_size;
-	struct os_info_entry entry[10];
-	u8	reserved[3864];
+	struct os_info_entry entry[OS_INFO_MAX];
+	u8	reserved[3804];
 } __packed;
 
 void os_info_init(void);

arch/s390/kernel/os_info.c

@@ -73,6 +73,7 @@ void __init os_info_init(void)
 {
 	struct lowcore *abs_lc;
 
+	BUILD_BUG_ON(sizeof(struct os_info) != PAGE_SIZE);
 	os_info.version_major = OS_INFO_VERSION_MAJOR;
 	os_info.version_minor = OS_INFO_VERSION_MINOR;
 	os_info.magic = OS_INFO_MAGIC;