vmap: cope with vm_unmap_aliases before vmalloc_init()

Xen can end up calling vm_unmap_aliases() before vmalloc_init() has
been called.  In this case its safe to make it a simple no-op.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Linux Memory Management List <linux-mm@kvack.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

mm/vmalloc.c

@@ -592,6 +592,8 @@ static void free_unmap_vmap_area_addr(unsigned long addr)
 
 #define VMAP_BLOCK_SIZE		(VMAP_BBMAP_BITS * PAGE_SIZE)
 
+static bool vmap_initialized __read_mostly = false;
+
 struct vmap_block_queue {
 	spinlock_t lock;
 	struct list_head free;
@@ -828,6 +830,9 @@ void vm_unmap_aliases(void)
 	int cpu;
 	int flush = 0;
 
+	if (unlikely(!vmap_initialized))
+		return;
+
 	for_each_possible_cpu(cpu) {
 		struct vmap_block_queue *vbq = &per_cpu(vmap_block_queue, cpu);
 		struct vmap_block *vb;
@@ -942,6 +947,8 @@ void __init vmalloc_init(void)
 		INIT_LIST_HEAD(&vbq->dirty);
 		vbq->nr_dirty = 0;
 	}
+
+	vmap_initialized = true;
 }
 
 void unmap_kernel_range(unsigned long addr, unsigned long size)