greybus: unlock gbuf mutex on free

To drop a reference on a gbuf, greybus_free_gbuf() is called.  That
uses kref_put_mutex() to drop the refernce under protection of
gbuf_mutex.  However the release routine, free_gbuf(), never
releases the mutex as it should.  Fix that.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/gbuf.c

@@ -82,6 +82,7 @@ static void free_gbuf(struct kref *kref)
 	gbuf->connection->hd->driver->free_gbuf_data(gbuf);
 
 	kmem_cache_free(gbuf_head_cache, gbuf);
+	mutex_unlock(&gbuf_mutex);
 }
 
 void greybus_free_gbuf(struct gbuf *gbuf)