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/20221023162533.81561-1-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/sysctl.c

@@ -2125,6 +2125,7 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO,
+		.extra2		= (void *)&page_cluster_max,
 	},
 	{
 		.procname	= "dirtytime_expire_seconds",