make get_file() return its argument

simplifies a bunch of callers...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
index c30f3e1..460e22d 100644
--- a/drivers/base/dma-buf.c
+++ b/drivers/base/dma-buf.c
@@ -460,8 +460,7 @@
 	if (vma->vm_file)
 		fput(vma->vm_file);
 
-	vma->vm_file = dmabuf->file;
-	get_file(vma->vm_file);
+	vma->vm_file = get_file(dmabuf->file);
 
 	vma->vm_pgoff = pgoff;