mm/mempolicy: fix get_nodes out of bound access

When user specified more nodes than supported, get_nodes will access nmask
array out of bounds.

Link: https://lkml.kernel.org/r/20220601093211.2970565-1-tianyu.li@arm.com
Fixes: e130242d ("mm: simplify compat numa syscalls")
Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mempolicy.c

@@ -1388,7 +1388,7 @@ static int get_nodes(nodemask_t *nodes, const unsigned long __user *nmask,
 		unsigned long bits = min_t(unsigned long, maxnode, BITS_PER_LONG);
 		unsigned long t;
 
-		if (get_bitmap(&t, &nmask[maxnode / BITS_PER_LONG], bits))
+		if (get_bitmap(&t, &nmask[(maxnode - 1) / BITS_PER_LONG], bits))
 			return -EFAULT;
 
 		if (maxnode - bits >= MAX_NUMNODES) {