dma-buf: Fix memory leak in sync_file_merge()

In the implementation of sync_file_merge() the allocated sync_file is
leaked if number of fences overflows. Release sync_file by goto err.

Fixes: a02b9dc9 ("dma-buf/sync_file: refactor fence storage in struct sync_file")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122220957.30427-1-navid.emamdoost@gmail.com