make get_file() return its argument

simplifies a bunch of callers...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/mm/mmap.c b/mm/mmap.c
index ae18a48..872441e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1301,8 +1301,7 @@
 				goto free_vma;
 			correct_wcount = 1;
 		}
-		vma->vm_file = file;
-		get_file(file);
+		vma->vm_file = get_file(file);
 		error = file->f_op->mmap(file, vma);
 		if (error)
 			goto unmap_and_free_vma;