spcom: use mutex on spcom_unlock_ion_buf()

Only the channel owner app can lock/unlock a channel ion buf.
However, if the app share the ION buf FD with its child tasks,
they might wrongly free the same ion buf twice.
The ion driver panic if a non-valid ION buff handle is provided.

Change-Id: Ia8166df5ea314949090f7e94e90eff3a3ed78b19
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>