staging: ion: refactor ion's dmabuf manipulators into a separate file.

This patch is preparatory work for making ion heaps modular. The patch
itself doesn't make any significant changes except for re-organizing the
buffer manipulator functions in a single file. This will be helpful
later when we specifically export some of these functions to be used by
a heap module.

Bug: 133508579
Test: ion-unit-tests

Change-Id: I52ecea78a179c936006a21beb7630009984cb22f
Co-developed-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Sandeep Patil <sspatil@google.com>

include/linux/ion.h

@@ -10,8 +10,18 @@
 #include <linux/err.h>
 
 #ifdef CONFIG_ION
-/*
- * Allocates an ION buffer.
+
+
+/**
+ * ion_alloc - Allocates an ion buffer of given size from given heap
+ *
+ * @len:               size of the buffer to be allocated.
+ * @heap_id_mask:      a bitwise maks of heap ids to allocate from
+ * @flags:             ION_BUFFER_XXXX flags for the new buffer.
+ *
+ * The function exports a dma_buf object for the new ion buffer internally
+ * and returns that to the caller. So, the buffer is ready to be used by other
+ * drivers immediately. Returns ERR_PTR in case of failure.
  */
 struct dma_buf *ion_alloc(size_t len, unsigned int heap_id_mask,
 			  unsigned int flags);
@@ -22,5 +32,7 @@ static inline struct dma_buf *ion_alloc(size_t len, unsigned int heap_id_mask,
 {
 	return ERR_PTR(-ENOMEM);
 }
+
+
 #endif /* CONFIG_ION */
 #endif /* _ION_KERNEL_H */