dma-buf: skip fd iteration for threads created with CLONE_FILES

All the threads in a thread group will share the same files_struct
unless it is not created with CLONE_FILES flags. Since most of the
threads will be created using the posix api, pthread_create, that
internally uses CLONE_FILES, we can skip iterating for the tasks that
share the same files_struct with the group leader in a thread group.

Change-Id: I84fc249ef37d7ebc55b59bc3b4ec4d3e6ace5165
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>