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

Commit 8d5796d2 allows for the vmalloc
area to be resized from the kernel cmdline.  Make sure it cannot overlap
with RAM entirely.
Signed-off-by: NNicolas Pitre <nico@marvell.com>
Signed-off-by: NRussell 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);