mm: use helper for calling f_op->mmap()

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/ipc/shm.c b/ipc/shm.c
index 81203e8..4329fe3 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -423,7 +423,7 @@
 	if (ret)
 		return ret;
 
-	ret = sfd->file->f_op->mmap(sfd->file, vma);
+	ret = call_mmap(sfd->file, vma);
 	if (ret) {
 		shm_close(vma);
 		return ret;