swap: add a limit for readahead page-cluster value

Currenty there is no upper limit for /proc/sys/vm/page-cluster, and it's a
bit shift value, so it could result in overflow of the 32-bit integer.
Add a reasonable upper limit for it, read-in at most 2**31 pages, which is
a large enough value for readahead.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kairui Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mm.h

···
 
 extern void * high_memory;
 extern int page_cluster;
+extern const int page_cluster_max;
 
 #ifdef CONFIG_SYSCTL
 extern int sysctl_legacy_va_layout;

kernel/sysctl.c

···
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO,
+		.extra2		= (void *)&page_cluster_max,
 	},
 	{
 		.procname	= "dirtytime_expire_seconds",

mm/swap.c

···
 #define CREATE_TRACE_POINTS
 #include <trace/events/pagemap.h>
 
-/* How many pages do we try to swap or page in/out together? */
+/* How many pages do we try to swap or page in/out together? As a power of 2 */
 int page_cluster;
+const int page_cluster_max = 31;
 
 /* Protecting only lru_rotate.fbatch which requires disabling interrupts */
 struct lru_rotate {