[ARM] prevent the vmalloc cmdline argument from eating all memory

Commit 8d5796d2ec6b5a4e7a52861144e63af438d6f8f7 allows for the vmalloc
area to be resized from the kernel cmdline.  Make sure it cannot overlap
with RAM entirely.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/mmu.c

@@ -646,6 +646,13 @@ static void __init early_vmalloc(char **arg)
 			"vmalloc area too small, limiting to %luMB\n",
 			vmalloc_reserve >> 20);
 	}
+
+	if (vmalloc_reserve > VMALLOC_END - (PAGE_OFFSET + SZ_32M)) {
+		vmalloc_reserve = VMALLOC_END - (PAGE_OFFSET + SZ_32M);
+		printk(KERN_WARNING
+			"vmalloc area is too big, limiting to %luMB\n",
+			vmalloc_reserve >> 20);
+	}
 }
 __early_param("vmalloc=", early_vmalloc);