dma-mapping: introduce a dma_common_find_pages helper

A helper to find the backing page array based on a virtual address.
This also ensures we do the same vm_flags check everywhere instead
of slightly different or missing ones in a few places.

Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/remap.c

@@ -11,6 +11,15 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 
+struct page **dma_common_find_pages(void *cpu_addr)
+{
+	struct vm_struct *area = find_vm_area(cpu_addr);
+
+	if (!area || area->flags != VM_DMA_COHERENT)
+		return NULL;
+	return area->pages;
+}
+
 static struct vm_struct *__dma_common_pages_remap(struct page **pages,
 			size_t size, pgprot_t prot, const void *caller)
 {
@@ -78,9 +87,9 @@ void *dma_common_contiguous_remap(struct page *page, size_t size,
  */
 void dma_common_free_remap(void *cpu_addr, size_t size)
 {
-	struct vm_struct *area = find_vm_area(cpu_addr);
+	struct page **pages = dma_common_find_pages(cpu_addr);
 
-	if (!area || area->flags != VM_DMA_COHERENT) {
+	if (!pages) {
 		WARN(1, "trying to free invalid coherent area: %p\n", cpu_addr);
 		return;
 	}