dmabuf: set fd memory to keep mapped

set fd memory to keep mapped.

Upstream Status: Pending

https://bugzilla.gnome.org/show_bug.cgi?id=768794
diff --git a/gst-libs/gst/allocators/gstdmabuf.c b/gst-libs/gst/allocators/gstdmabuf.c
index 170267b..a9a0775 100644
--- a/gst-libs/gst/allocators/gstdmabuf.c
+++ b/gst-libs/gst/allocators/gstdmabuf.c
@@ -100,7 +100,7 @@
 {
   g_return_val_if_fail (GST_IS_DMABUF_ALLOCATOR (allocator), NULL);
 
-  return gst_fd_allocator_alloc (allocator, fd, size, GST_FD_MEMORY_FLAG_NONE);
+  return gst_fd_allocator_alloc (allocator, fd, size, GST_FD_MEMORY_FLAG_KEEP_MAPPED);
 }
 
 /**