Recreating our context's syncobj with ALREADY_SIGNALED meant that if you
created two fences in a row, then waiting on the second would succeed
immediately.  Instead, export a sync file in the gallium fence (since we
don't have a syncobj clone ioctl), and just create a new syncobj to wait
on whenever we need to.

Noticed while debugging
dEQP-GLES3.functional.fence_sync.client_wait_sync_finish