staging: ion: export ion_free() for ion_heaps.

ion_free() is a generic function needed to release
an ion buffer back to the heap. So, export it for the
heap module.

Bug: 140294230
Test: ion-unit-tests

Change-Id: Iaa467ab3df18fa219412dd9f5913cd022a93455d
Signed-off-by: Sandeep Patil <sspatil@google.com>

include/linux/ion.h

@@ -293,6 +293,13 @@ int ion_buffer_zero(struct ion_buffer *buffer);
 struct dma_buf *ion_alloc(size_t len, unsigned int heap_id_mask,
 			  unsigned int flags);
 
+/**
+ * ion_free - Releases the ion buffer.
+ *
+ * @buffer:             ion buffer to be released
+ */
+int ion_free(struct ion_buffer *buffer);
+
 #else
 
 static inline int __ion_device_add_heap(struct ion_heap *heap,
@@ -357,5 +364,10 @@ static inline struct dma_buf *ion_alloc(size_t len, unsigned int heap_id_mask,
 	return ERR_PTR(-ENOMEM);
 }
 
+static inline int ion_free(struct ion_buffer *buffer)
+{
+	return 0;
+}
+
 #endif /* CONFIG_ION */
 #endif /* _ION_KERNEL_H */